Here are some things that I am considering as I am trying to sort out this riddle for myself.
1. Once the pole starts to bend, the athlete can completely let go with the bottom hand, and if they have enough hand strength to hang on, the loss of the bottom arm will have no influence on the action of the pole.
2. On the other side of the coin, pushing with the bottom arm cannot hurt penetration either, provided the athlete has achieved a free takeoff. They are already off the ground, so the pushing action cannot be forcing them back down the runway, as it would if they were low and under and using the left arm to try to bend the pole while their takeoff foot is still on the ground.
3. In my own experience, although my bottom arm was almost straight at takeoff, I only felt the briefest sensation of my left hand hitting the pole. It was not as though I was straining to keep the pole away from me. My left arm only impacted the pole slightly, almost passively, and then I was pulling with it as hard as I could. And while I was pulling, instead of bending, my arm actually straightened more as the bend of the pole pulled my left hand up and away from me. When I appeared to be pushing on the pole in the middle of the vault, I was doing the exact opposite.
4. When I teach the Petrov pole carry, approach and takeoff to my beginners, they instantly start bending the pole and easily clearing bars with their bottom arms completely collapsed at takeoff. There is an astonishing sequence of photos on page 50 of the first edition of Beginner to Bubka of a girl executing a beautiful vault. At the takeoff her left arm is bent so much that her elbow has actually passed the pole. Her left hand is very nearly touching her right ear. This photo, more than anything I have read or heard, turned my vault world upside down. By everything I understood at that time, that jump from that position at takeoff was impossible, yet there it was.
5. In the beginning of his career, Bubka's bottom arm collapsed nearly as much as the girl in B to B, and he jumped with his standards very close to the box. On most jumps his bottom hand nearly touched the top of his head with the left elbow bent out to the side at a ninety degree angle. Later in his carreer, however, his arm was much straighter at takeoff, and he jumped with the standards farther back.
The upshot of all of this is that I am certain that the bottom arm has nothing to do with beginning or maintaining the bend of the pole. I am also pretty sure, and the interview with Yelana bears this out, that the bottom arm can play some role in the beginning of the vault, and that role changes the bend of the pole and enhances its movement. (I am aware of the apparent contradiction in those two statements, and this highlights the level of confusion and complexity I am struggling with. There is, however, a difference between "beginning" and "maintaining" and "changing" and "enhancing.") I am also pretty sure that this action may begin before takeoff, but only impacts the pole immediately afterward. This influence is slight; remember we are talking about an improvement for a vaulter who is already a multiple world record holder. Obviously, you can beat everybody on the planet without the left arm doing anything at all. So the question remains: if I know what the left arm cannot do, what remains that it CAN do?!
Right now I am just letting all of this percolate around in my mind as I watch and teach and gain experience. I am starting to formulate the shadow of an answer, and it is coming to me along these lines. The bottom arm cannot (in a good jump) be used to make the pole bend, but it can alter the shape of the bend, and It can change the position of the athlete's hips relative to the pole. When the bottom arm is collapsed, the chest and hips come very close to the pole at takeoff, and then as the athlete swings, the pole moves abruptly away as the pole bends and pulls the left hand up and out. Also, the pole bends less acutely at takeoff. However, with the bottom arm straighter at takeoff the athlete's hips start behind the pole and stay there as they begin to swing. The pole also bends more after takeoff with the vaulter's left hand rising significantly above the right.
This is the best video I have found to illustrate what I am talking (incoherently) about. http://www.youtube.com/watch?v=-b8OKuHLR68
The music is horrible; I just mute it. The vaults at 0:08, 1:57, and 3:19 are early Bubka and typical of what we saw him doing in the '80s. The vaults at 1:47, 2:56, and 3:13 are late Bubka, and while almost every area of the jump is unchanged, there is a definite difference in his bottom arm. Could this be the change Yelana is working towards? Once again, I still don't know. I'm just thinking out loud.