I've referred to this before in other posts (sequenced passivity and golf's triangles), but it also ties closely into other posts and fundamentals that did not mention Els, including the right hip-right elbow relationship and arms falling to start the downswing (as Shawn Clement's "arm-club unit falling down the plane").
Simply stated it's this: the right elbow, right forearm, and right wrist (and thus the club) form a box shape (actually more of a rectangle with 2 90-degree angles) at the top of the swing. During transition, one simply allows this box shape to remain intact and fall down in front (right elbow coming close to the right hip while the shoulders REMAIN turned) and then release it past the ball. As Ernie so simply puts it, "Keep the box and give it a lash!"
I believe it's probably best to focus on the right arm only in the backswing, because doing this properly will keep the left arm straighter and help set the wrists earlier. And thinking of the box shape in the downswing is a given; maintaining it as long as possible gets 2 crucial levers working in the swing: the right elbow and right wrist straightening in the right sequence and at the right times. This adds tremendous, effortless power to the swing (for all shots…include bunkers and pitch shots).
This is Els' way of describing what TGM'ers call the use of Pressure Point #3: "Keeping the box" as Els describes will place pressure in the base knuckle of the first digit on the right hand from the start of the downswing until past impact. This is often referred to as "lag pressure," and it's maintained all the way down into the ball. As a matter of fact, one can think of the base knuckle or first phalanx of the right finger as analogous to the club face; it's often called the "trigger finger," though most of the finger should do NOTHING during the swing.
By using this "box" method, you will notice that the left and right wrists can cock up and down WHILE the right wrist remains hinged backwards, and it's this backwards hinged right wrist that creates a flat left wrist at impact, maintains lag pressure in PP #3, gets the hands leading the club head, compresses the ball, and creates a forward leaning shaft. A key is to start down or transition slowly; the greatest speed is added at the bottom, not the top.
In reality, Ernie could have called this "Avoid the cast!" instead of "Keep the box!" Casters apply an active pushing force, unhinging force, and/or uncocking force (either consciously or unconsciously) at the top (and thus have to flip the wrists at the bottom), while laggers (i.e., good players) form the angles at the top and then drop that arm-club "power package" intact from the top (the right wrist remains hinged backwards through impact). Thus, the angles formed in the right arm during the backswing are retained late into the downswing. Casting is often associated with an early shoulder rotation; the arm-club power package should ideally begin dropping in response to the lower body BEFORE the shoulders open up.
Many instructors will tell you that maintaining lag pressure is not something you can do actively, but that poses the question: Is consciously fighting the urge to cast considered an active attempt to maintain lag? One could argue that "keeping the box" as Els describes is a means of actively keeping the angles and lag pressure in PP #3. Martin Hall's take on this is to "keep the thumbs pointing up in the air as long as possible," which also implies an active effort to maintain lag. As with all things in golf, let your results be the arbitrator.
A good way to practice this move is using the Pump Drill (see videos below); Hogan doesn't refer to it as the Pump Drill, but you can see him demonstrating it in his video. Another drill advocated by Hogan and Brian Manzella is to simply remove the right thumb from the shaft and hit some balls (Sam Snead used to remove his right thumb and right index finger entirely to practice). There are also drills that advocate holding a wet mop like a club (or using a club with a towel on the end) and dragging it across the ground. An impact bag is also used to instill the correct feeling at impact.
My take on the Pump Drill is to completely remove the right hand thumb and index finger from the handle to accentuate the pressure feeling in the base phalanx of my right index finger. From the top of the backswing, use the left hip motion to cause the arm-club unit to pump up and down a few times. The end of a "pumping action" gets the hands over the ball with the club shaft still parallel to the ground. After one or two "pumps," let it go at the bottom.
What happens when lag is taken too far? Skull shots (not really thin shots) because the club bottoms out late and balls that fly off to the right because the club face doesn't close in time. Rejoice if you see these occasionally instead of weak fat or thin shots that come from casting and flipping.
It's very crucial to keep your head behind the ball (i.e., maintain a tilted spine angle); just get the feel that once you get over the right leg in the backswing that the head stays there while the lower body weight shifts to the left. Keeping the head back assists with shallowing out the swing and with hitting inside-out.
Even cooler, if you "shake hands" with your right hand towards the target, you can coax a draw or straight-ish ball flight (the toe of the club overtakes the heel), or you if you hold off the right hand somewhat (toe and heel come through more together) you can get a fade.
As Ben Hogan rotates his lower body towards the target, the "straight left arm + bent right arm" act as a single unitary structure and they are both passively pulled down towards waist level as a single unit. Homer Kelley refers to this unitary structure of the left arm and bent right arm as the power package assembly . In other words, Ben Hogan does nothing actively with his hands/arms/club during the early downswing - he simply allows them to respond reactively/passively to the shift-rotational movement of the lower body, and he simply allows the entire power package assembly to be passively pulled down to waist level.
[Drill for learning the feel of lag pressure.]
BM: "Grip the club normally. Take the right thumb off of the club and point it at the target. Hit it."
[Why does this work?]
Forum Member: "...when most of us begin to lose the lag, we compensate by feeling/adding pressure from the right thumb to try and stop the throwaway....without the right thumb on the club, we must store and hold the lag or be ready for the club to flop around coming into impact (especially if there is throwaway or if we lose the lag on PP#3)."
BM: "Helps Slicers too!
Forum Member: "Without the right thumb exerting pressure on the club, one can drag the sweetspot onplane via the right index finger, thus closing the clubface by impact."
In the Golf swing there are 4 distinct pressure points (more on that in another article) but again, for simplicity stakes, we will focus on the easiest and more convenient to feel: the meaty part of your index (the "trigger finger") of your trailing hand where it touches the club.
Here is the trick: Are you able to perform a Golf stroke while feeling pressure in this index from start down to both arms straight (the end of the follow-through and before the finish)???
Even better: if you can feel this pressure not decreasing (unrelentless pressure), I bet my shirt you must be a 1 digit handicap as you must have Lag in your swing with all the good things that come with it!
A player who casts the club (club head throwaway) does exactly that: he feels a lot of pressure in the start of the downswing (by accelerating the club very hard and convulsively). Such a sudden accelartion thows the club away with no chance to catch it up before impact.
Needless to say, that player feels no pressure at all in the index finger when the club head arrives at the ball.
The correct way to do it is to push your cart evenly with no over-acceleration.