When someone speeds up his Taiji form, how can you tell whether he is training Taiji or not?
You demonstrate a fundamental lack of understanding over what Tai Chi is and how to train it. The reason for the slow form is not for shits and giggles. it is because you are attempting to be introspective *(the wu de of min) throughout the form. Speeding up the form only makes that more difficult.
The form itself, the long form (or first form) has been intentionally modified to support it being slower. You cannot merely speed up the form. The idea of speeding up the form in fact, itself, does not make any sense. It is based on the mistaken notion that speeding up the form will make the techniques work in a real fight -- as if you are 'training for speed'. This is wrong.
When discussing how the actual techniques are to be applied, in fact the form itself is often completely changed when applied. A great example of this is shown in the following videos:
1. Practical method
You can see several different applications of this move shown in the video; note: 4.35-4.45: "That's why the application of this move is actually
here."
Now, let's cross reference with Ma Hong, note at 0:30:
2.
Ma Hong's application is completely in the opposite direction than CZH's!
And thus we see why focusing on speeding up or focusing on applications can become a dead end for many people. When you
remove utility and train fundamental techniques, the gong (or truth) of Taijiquan comes out. This is why we train slow in the first place. Because we're not trying to train speed. Speed is of the mind. When the mind moves the body will move (yi leads qi). This can be extremely quick. Much quicker. There is no need to train to speed up the form. Adding in a few fajing movements during special practice (i.e. a second form). or
sparesely though the first form is more than enough.
Thus it's easy to know whether someone who speeds up their Taiji form is training Taiji or not;
if they speed up the form, they're not training Taiji.