How many times have we heard this cliche 'early detection is the best protection', particularly in reference to health which it is vital for existence. Although we all agree this is the best medicine we seldom follow this course of action until it is too late because we let other trivial things get in the way which, at the time, seem more important.
This cliche also means something in Jiu Jitsu; it explains why some techniques work and others don’t. It is a matter of detection in the early part of the piece that something bad is going to happen if we don’t do something about it When the martial arts were being taught, during the era of black and white television, self defence was usually based on reacting to situations usually when it was too late. By this, I mean, eg rear naked choke; most people today know it's impossible to escape once it's put on tight so it is absurd to show a student how to get out of it. It's a bit like expecting to get out of a stray jacket and handcuffs; or similarly, trying to block a punch when it is 2cm away from your face.
Students should be trained to put their plan immediately into action at the first sign of a threat; that's the best medicine. Those who stand out in front of the class who think they can do the almost impossible lack reality based experience. Anything else is pure entertainment.