Balking: what is the reason? One possible reason is what Ken already mentions, an inconsistent run up. When you come a bit too close you suddenly see that bar so close and balk. A hint: try to forget the bar completely. When you have a good run up you will always be able to jump properly. I once saw a blind high jumper. He felt with his hands where the bar was, walked backwards, did a run up and jumped with good technique. So the whole movement was in his head – and we can try to learn that too. Some jumpers start with watching the bar and during the complete run up remain fixed to the bar. But it is better to forget the bar, do the straight part of the run up, concentrate on the circular part of the run up, do all necessary take off actions. And I promise: you will pass that bar without ever having seen it.
Another cause of balking could be fear. Fear that you do not pass the bar and will fall upon the bar. Well, it hurts, that's right. But it heals and you can forget it... Here the hint is again in a good run up. When the last stride of the run up and the take off actions are OK you'll never fall upon the bar, even when it comes down!
Here the sitting comes in... Part of that is doing the reverse of arching. But most likely the sitting has been caused by wrong take off actions. An important thing is forgetting the stretching of the hip joint. Sitting jumpers often stretch their ankles and knees, but forget the third joint, the hip. (In the scissors technique the hip is not stretched; often a bad flop results from doing scissors.) With a good stretched hip at take off the line from head to foot passes behind the body, with an unstretched hip that line passes in front of the body.
Sitting always goes together with a low amount of rotation around the bar. But when you have enough rotation you'll never have to be afraid of falling on the bar. Rotation is caused by three things: a proper curve in the run up, swing leg action during take off, stretching of the hip. The intention of the curve is only that during your last stride you come from hanging into the curve to an upright take off position. The swing leg should swing a little bit away from the bar. The hip stretching I have already mentioned.
When I look at a video recording of a jump I watch the movement in real time to judge the dynamics. But then I look up only three frames: the mid of the last stride (the last time your centre of gravity (cg) passes above the swing leg foot), the position at the end of take off (last ground contact), and the highest point of the cg above the bar. See some of my pupils in 2006 at
http://www.at-a-lanta.nl/weia/hellas.html
The first two frames contain nearly all information that is needed to predict the position above the bar! The third frame is only needed to see whether the predictions were good or not...
Marije has a good arch but more important a good amount of rotation. She causes that mostly with her swing leg, secondly by coming upright during the last stride, the hip stretching is not really good.
Erik is the next. Good roation caused by his coming upright (watch how he leans into the curve), excellent swing leg and pretty good hip stretching.
Have a look at Jelle. Poor rotation, and that is fully the result of wrong hip stretching.
Marlies is interesting (now a good 800 and 400hurdles runner). Quite a difference in the amount of rotation in both jumps, in my opinion explained by the difference in hip stretching.
Then Robin. See how he does not stretch his hip, the line from shoulders to foot passes in front of the hips. Compare with Erik. Important also is that he does hardly lean into the curve.
Jumpers with good rotation (Marije, Erik and some others) will inevitably have a miss sometimes, but will not fall upon the bar, they in fact are already behind the bar when it comes down. Jumpers with bad rotation will touch the bar at the front during a miss, take the bar with them and maybe fall upon it.
That's all...
)) Sorry for so much text.