We were going through kicks in tonight's class, one of the new guys asked how to block a roundhouse kick, and Sensei showed us a block that we (white belts) have never learnt before. One of the red belts recommended that we could use Gedan-Barai, which is probably what I would've said too considering the exceeding limited repertoire that we've been taught, but Sensei disagreed. IMO neither the block she showed or the Gedan are better or worse than the other... and really, kicks are better off being blocked with legs than hands/arms (which you would use only if the kick's come in too fast for you to get your leg up in time - so arm blocks against kicks are still useful IMO as a backup). I was talking to that red belt later and asked him if he's ever been kicked by someone with a massively powerful roundhouse kick (like say, a lot of Muay Thai fighters)... cos blocking those with your arms would
cane! 
(even blocking them with your legs hurts, but I'd hate to imagine how painful they'd be to block with arms!)
GKR kicks are similar to Tai Chi kicks insofar that the knee is lifted first before doing the kick, which sacrifices power, but gains greater manoeuvreability and versatility. Other fighters launch their kicks straight from the ground without lifting the knee first. While they sacrifice manoeuvreability and versatility, they gain massive power! XO Although I must say that one significant difference w/ the GKR roundhouse kick is that the knee is lifted up to the
side instead of in front like with a front, side and back kick (in Tai Chi we lift to the front, same as every other kick). I can see that this is to help give the roundhouse kick more power, but
damn it _completely_ opens the groin!!

If you wanna generate more power, I reckon you're better off just launching the kick straight off from the ground and forget about the knee lift. Lifting it off to the side when kicking straight ahead kinda defeats the versatility advantage since you're exposing yourself anyway.
