I attack my Sensei (When demonstrating a technique) pretty much the way anyone on the street would. Not because I want to prove a point, or that I want to test my Teacher, no.. just because, sometimes, you have to, It helps build more confidence, it refines your technique, and enhance the senses. Doing Aikido slow and secure all the time is great, but sometimes, speeding up things really give different results.
I also do surprise attacks on my Sensei (and other Aikidokas) in the middle of our practice, it's sooooo much fun!