So what you are looking for is the full big picture. It occurs with the front knee hitting the floor and its relationship to the bowling arm coming up. When your knee just touches the ground your bowling arm should be around shoulder height and straight back. So between 1 and 2 the straightening of the knee happens to get through the crease -- or between 3 and 4 give or take for Curtley.
I think what you are talking about the bounding up and the upsetting of the rest of the rhythm comes from too early use of the bowling hand. I had a discussion with Chippyben last year about using the bowling arm later. It could also be able getting enough power through your back leg to get through the crease so you are not going up -- but through. This is something of using the centre of your back foot.
If you are having trouble then it is probably because of one of these two -- however it's probably difficult to know without looking at the action. Something of a lesson for me probably -- to post up images properly hehe.
Wow! you hit the nail on its head. I was just discussing with a fast bowler in our club about the bracing of the knee and when he couldn't understand what it meant, I tried to show it to him. In that process, I realized that the front knee is braced only if your delivery stride is long enough to be pushing off from your back foot - ( A parallel that batsmen might understand would be a front foot cover drive - bent front knee to lean into the drive, fully extended back knee to get as far down the pitch as possible). And I was just about to share my discovery here when I see that you have already diagnosed the problem.
But now that we have got to the root of the problem, I feel that I'd rather not change the length of the delivery stride and the way I go through the crease all at once. I have to admit it feels like letting go of my security blanket - a short delivery stride might not get me as much power into the action, but it allows me to pivot around the front leg which I feel is central to getting enough revs on the ball.