Thanks lads, the only reason i ask, is that it hurts my kneecap on anything other than mats (even mats a little) is that because i'm not bringing the other leg through fast enough, causing a harder hit? a level change issue?
The idea of dropping your knee is that it allows you to get deep underneath your opponent, and get in on his hips. Now, some wrestlers do that without dropping their knee-- but the reason you drop the knee is to really force yourself to change levels and explode up and into your opponent. Some coaches will fucking scream at you if you shoot without dropping the knee. You wanna be knocking the bro over with your drive and shoulders already, drop knee or not.
Now, sometimes people drop the knee and don't drive forward. If your knee hurts from dropping when you shoot, it's possible that you're just dropping your knee, and then popping up. You should be driving forward and into your opponent as you drop the knee, and driving the whole time (though the direction can change, depending on if you're cutting the corner or not, or your lift, etc.). So, dont just fall down onto your knee-- your knee should be skimming the mat as your other leg comes across to get an angle on your opponent.
I always, always drop my knee. I fucking drop my knee for single legs a lot. I love my shot when I drop my knee. Now, for MMA? If I ever want to do MMA, I'll probably try and work my shot without dropping my knee. Also, streetfights (if that's your cup of tea) you don't wanna be dropping your knee all the time. I try my best to not be wrestling with dudes in bars or on sidewalks, so I don't care about the latter. But there you go.
edit: ^^^I fucking drop my knee as I get out of bed in the morning. Cereal? I drop my knee and explode up to get those fucking Fruit Loops.