They only learnt about this because of shenanigans while trying to get a wu that somehow ended with jack picking up raimundo effortlessly and lifting him up. which made everyone freeze in surprise including jack himself as he slowly comes to realise that rai weights nothing to him.

Dojo realises whats going on as the kids try to understand what the hell is going on, which includes everyone lifting rai (to his dismay) and actually putting him on a scale (luckily his still average weight)

Dojo explains to the kids that this can happen at times when one is really connected to their element, and that this manifestation is probably what helps rai be able to use wind to fly.

rai asks if he can turn it off.

dojo says he doesn't know, the xiaolin order usually tries to go the safe route when it comes to how they train elemental abilities.

raimundo dreads the day that they/his has to go to his family cause they will take note on it and they will have a talk before raimundo can actually explain the situation to them.