Last night we ended class running through a series of sticky hands drills, with a focus on #3, the figure-4 and a couple of its variations. This technique works to control an opponent who is attacking with an overhead strike, either a hammer fist, or with a knife.
Here is the exercise as a straight sticky-hands drill.
The same exercise demonstrated with a knife.
Finally, a knife defence tutorial would not be complete without this demonstration, which in my opinion is easily the single most effective defence against a knife, by far.