Overall, good form and speed. I like it when people post videos of their technique because I'm always looking for ideas to improve myself.
The only thing is, a lot of times from a forum you'll get advice that may be conflicting. For example, every time a guy posts a video of his roundhouse one of the first comments will be that you shouldn't drop your kicking arm, even though this is the orthodox way of throwing the kick.
The slapping sound seems to be partly due to the kick not landing square on the center of the pads...it seems your friend isn't that experienced holding the pads and bracing for the force of the kick, so that might explain it.
My 2 cents:
1) About dropping the lead hand prior to kicking. I think this is actually a legit way to kick. If I'm really trying to get power/torque into my kick, as I'm stepping out I'll first yank my lead hand back and bring my kicking arm up, then as I execute the kick I'll bring my lead arm up and across my chin as a guard and whip my kicking arm down.
Does this telegraph a bit? Of course. Note, though, that the above movements are done very quickly, when you are stepping out at 45 degrees. I feel that this really adds to the torque in your roundhouse (you're loading your hips in a sense).
I've noticed that a LOT of top pros do this - Mirko, Buakaw...heck, these two don't even bother to bring their lead arm back up sometimes. This is, of course, risky (Randleman showed that to Mirko), just like dropping your rear arm is, so I also practice throwing quick kicks and consciously keeping my lead arm up.
Another thing I do to add power and really pummel the bag is this (I'd like to get some advice on this too):
Before I step out to throw the kick, I'll "pop" my hips out a bit to the right (orthodox stance), with a slight pivot of both feet to the right (this is a quick skipping movement)...then I'll step out at 45 degrees to the left to throw the kick. This is just one of those things, along with dropping the lead arm, that "feels" right to me...it seems to add torque. I've seen Samkor do this quite a bit...of course, he doesn't care if he telegraphs because he'll hurt you even if you block him
. I only do this when I'm training power, though.
