Yeah, if they try to hook knees, in an mma ruleset you can just inside leg sweep them and take top position on the ground. He hook knees with his left knee, then you step in with your left leg on his right for the trip, and slam/takedown. Very natural, and easy counter.
thx for the replies what if your Not in the clinch, but close punching distance and he throws a knee to the head.. through your guard. do you just have close the distance with your hips? see the knee coming and move your head? Within punching distance and ive seen many Fighters get ko this way though heres a video of what i mean [ame]http://www.youtube.com/watch?v=ZV2RfVA2UPA[/ame]
Nothing specific with that situation that I can think of; just the normal stuff, counter when you see him start, move, just like you would with any other strike. A well timed right hand is a good choice ala Fedor vs Arlovski