First ... to the last 4 posters above ... you know who you are ... I completely agree. Had I read this 21 months ago ... or even 10 months ago ... I would not have understood your point about pausing in the C position. But today, I definitely know this to be a serious flaw. Any pause in the action breaks the "continuous chain" rule.
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Jeff, I'd particularly like to comment on your pic of Tim Mack.
He's doing 2 things that are "questionable" ... IMHO.
1. He's blocking out with his bottom arm.
2. His trail leg is bent at almost a 90 degree angle ... like Skipper.
#1 is by intent, as he intentional uses a "drive vault" technique. That's OK ... it's worked well for him ... in his situation. Who can argue with an Olympic Gold Medal?
I don't think it's a flaw of his ... I think that it's a flaw of mine ... to not understand how the hell he vaulted so high with this technique. We've been pondering this on the "Non-Petrov" thread for awhile now, and still no OBVIOUS answers or lightbulbs flashing on in our minds
in answer to that puzzle. I think Tim held a technical secret ... we just don't know what that secret was ... and it's not in his book either. 
On #2, I think I can be more critical, because I have more personal experience with how the trail leg MUST swing in order to optimize ANY technique ... be it the Drive Model or the Petrov Model.
The best way I can critique Tim Mack's trail leg swing is to compare it to Tim McMichael's swing. They're both so-called "drive vaulters", yet McMichael had a much more pronounced trail leg swing, much more elastic, much straighter than Mack .. and probably much quicker (altho that's just a guess right now - I haven't timed them).
Certainly ... if Newton's Second Law of Motion says that FORCE = MASS x ACCELERATION ... then McMichael adds more FORCE into his system due to his "more powerful" downswing ... because his trail leg has more acceleration at the Whip for his mass.
The difference, I think, is that when you allow your trail leg to bend like Mack, you allow energy to leak out of "the system". Not only that, but you allow your hips to swing forward WITH LESS POWER left over for a really STRONG, QUICK DOWNSWING.
I think the FORCE of the DOWNSWING is proportional to the acceleration that you can generate up to and including the Whip point.
VELOCITY = DISTANCE/TIME ... so between the 2 video frames just BEFORE your Whip and ON or just AFTER your Whip, the more distance your trail leg ... fully extended ... travels ... then the faster it's going ... which means that the more it has ACCELERATED from the somewhat ZERO speed it had directly after takeoff. Then multiply that acceleration by the weight of your trail leg, and you have a higher FORCE value.
My apologies for any slight formulae errors I might have made ... I'm not a physicist ... but this sounds about right. Please correct me ... if you can.
If the downswing (of your trail leg) isn't powerful enough, then you lose your momentum (force ... of your entire body) in the UPSWING, and are consequently left with no other choice but to "muscle-up" into a tuck ... and then shoot out of the tuck.
I don't know how Tim Mack jumped so high with his technique ... but I'd sure like to know. There's a secret there somewhere.
Kirk
