We practice techniques in class--chin-na for example--and we do it in a way that won't hurt our training partner. If we go too far, we can snap something and our partner won't be able to train for a while.
Unfortunately, a lot of beginners who haven't developed the right self-control tend to apply too much force too quickly and can hurt the people they're training with.
The result is a watering down of techniques. Too often, an arm bar is practiced as a technique that causes pain and puts your opponent in a bent-over or on-the-ground position of vulnerability. The same happens when practicing a wrist lock such as the one we call "half-moon" against a grab. Enough pressure is applied to cause pain and that's it.
In reality, if you get into an actual fight, it's a very serious matter. Someone is likely to need a trip to the ER at the end of the fight. At this point, you certainly don't want to worry about hurting your attacker. In fact, you want to hurt him quickly before he hurts you or the people you love.
That's why you need to adopt the proper mental intent when practicing techniques in class.
Too often, I see students punching or kicking with little force and with bad technique. They stick a punch out there. There's no snap, no body mechanics that would deliver power through the technique.
Every technique you throw in class must be delivered with the same power you would use if your life was in danger. When you do a chin-na technique, the point of chin-na isn't to cause pain and force your attacker to give up. The point of chin-na is to break your opponent quickly and stop the attack.
Every martial application in our forms is designed to end the fight. Every tai chi movement is a fighting technique designed to break an opponent quickly--instantly--and leave him broken on the ground while you walk away.
That's why I encourage my students to use the proper body mechanics and to deliver force without hurting your training partner. If I do an elbow break on someone in class, I deliver it with force but I stop before I make contact with the elbow. I pull the opponent's wrist back toward my chest at the same instant that I deliver force outward toward the elbow.
I'll put a video together to show what I'm talking about. My students should already know. And when you practice with a partner, if they aren't delivering force with their techniques--if their technique isn't strong enough to break a board or two, for example--you have to call them on it and correct each other.