It's not a pretty class where people use sweeping Hollywood-style blocks or somersualt over their opponent's heads. In our drills, we learn to be aware of how our blocks and our fighting stances align up with the attacking movement as we slash, stab, and slice towards each other (with practice knives, of course). Sometimes, someone gets poked, or screws up a move and misses a block, but it's all part of the learning process. Ultimately, the knife defense classes are a great way for students to visualize different knife attack scenarios and build confidence in their movement against such violent attacks.
Please take a look at one of self-defense classes by Sifu Lawrence Hill:
Part 1
Part 2