A mudra is a position. If you hold your hands in the Amitabha position of equanimity, then that would be a mudra. Once you release your hands, then that mudra is no longer there. If you sit in a Vairocana posture, that would be a sitting mudra. The moment you stand up, that sitting mudra isn’t happening anymore. Any position that you keep is a mudra.
If things solidify into certain forms, then those too are mudras. Any mudras done intentionally are an expression of the seventh ray of Ceremonial Order. If the position of your body is always moving and constantly changing, there is no mudra. Moving nonformally with no particular pattern would not be mudra. Any repetition of a movement could be considered mudra because there is a certain shape of the body being exhibited.
The position of your mind is a mudra. The Mahamudra is the great mudra. It means your mind is expansive and unlimited.
For materialists, mind doesn't even exist. They have been told that material objects are important and things that are subtle or intangible are not important. To them, the position of their mind is insignificant.
Anything that fixates is a mudra. But what if you simply stayed open? Then that would be the Mahamudra. An utter lack of fixation is the Mahamudra.
Mahamudra means the ultimate position. It is the perfect meditation posture: the perfect position, location and situation. When a beginner hears this, they think of sitting with their back straight, folding their legs and holding their hands a certain way. But here we are speaking of the mudra of your mind. When you assume this posture, then all passing stains become purified.