Usually when you see that, it's because they are using the wrong technique against the wrong attack. If the technique is done properly at the right time, and against the right attack, then you shouldn't have to do any "forcing." An example would be the video of when I took my sparring partner down with one hand and everyone said that it looked too easy and thought that it was more of my sparring partner being clumsy and not the technique actually working. The only reason the technique made it look that way was because it was done at the points of maximum efficiency. Everything from start to finish was done correctly, at the right time, with the right movement,and the right force.
The concepts of many martial arts system is not to use force against force. When you jab at a person's face then all of his force and energy is going forward in a straight light. Force is not being sent left or to the right. Because of this it will take very little energy to move the fist left or right. You can redirect the punch with hard or soft techniques. This is the fundamental mechanics of martial arts regardless of the system.
If you see that someone is forcing a technique then they are either trying to apply the wrong technique to the situation or they are not applying the best technique to a situation. There are many things that can turn the "right" technique into the "wrong" technique. The most common is if you start a technique too early or too late, which often means the practitioner has missed the window of maximum efficiency and is forced to "muscle" the technique. The other common scenario is when you opponent keys in to what you are about to do and changes his or her actions, which in turns makes the initial technique the "wrong" technique. Fighting is fluid so all of these changes happen quickly and the slightest change can be the difference between "super easy technique", to "man I really had to work that technique."
The fact that someone muscles a technique or uses weight does not mean that the technique isn't valid.