Reading an opponents movements and creating a counter to beat that attack is a very large part of being a successful, high level fighter. If you can sniff out an overly aggressive weapon being thrown, and you know the trump card to that weapon, then you will consistently be on the good end of exchanges.
In this video Bobby covers how to take advantage of a fighter who is overthrowing a right hook. First he reads this right hand a few times while only putting up one defence, which in this hand is a left handed high block. His opponent now feels comfortable with this offensive weapon and feels it is working as he is lulled into a false sense of security.
From here he pulls his lead foot back a tiny bit to create space for his rear knee. The beauty of this movement is that the attacking fighter throwing the overhand is actually creating most of the force, as they are moving all of their bodyweight forward.
A mistake you will see a lot of fighters make here is to drop their defence while throwing the knee and getting caught with the right hand anyways. In that instance they would be off balance while only on one leg, which can easily result in a knockout. So remember to keep your lead hand up beside your face to block the right hand and use your rear hand to grab onto your opponents head or shoulder to pull him into the knee.
As a general rule fighters should remember that punches beat kicks, and knees beat punches.
For more in depth analysis check out The Ultimate Guide to Striking for MMA, Muay Thai and Kickboxing.