all the intra-abdominal pressure caused from hoisting the weight up from that position, each and every rep. to work the muscle effectively, you should only be working with the top 80% of the movement anyway, always keeping a slight bend on the elbows, esp at the bottom. it is far more effective because you dont lose tension on the muscle at the bottom, like he demonstrates here each time. Also, I think the straight bar sucks for curls, especially heavy ones. Too much pressure on the wrists IMO..very awkward movement.
i cant help myself.
if you want to give advice, make sure it's sound.
first off, the top 80% is completre BS. in a free weight exercise, much of the force is gone at about the 70% up position as the moment (the horizontal distance from the pivot to the weight) is very small.
The "weight" that the muscle sees, is in fact, the moment. This is the case for EVERY exercise. So, the maximum moment in every exercise is the position where the weight is at a max from the pivot- where it is perpendicular to gravity.
So, if you want to maximse your energy, you need to be working about the max moment..say 30% each side, maybe 35%. i.e. no rest at the top, no rest at the bottom.
the problem is that this is very intense. Rest pause- which is a type of what this guy is doing, is completely fine. Also, as he swings to bring the bar up from the position of least mechanical advantage (where the weight is in line or behind the pivot AND against gravity), then this is considered ok also. Arnold built some very nice biceps using this form intuitively.
Now, the exception to this is with cables. You need to think of what "plane" the weight is coming from with them.