The idea behind Psionics is that you can replace Source Tags with Tags from other classes when Casting Magic. So in your example, using Psionic 2 to activate Guile 1 from the Face Class, you treat the Guile Tag as if it were a Source Tag, and apply it to the Cast Magic Move instead of applying it to Influence Someone.
The way I imagine it is telekinesis. If you’ve got mind powers, then you can lift up a sword and swing it around with your mind, where a soldier would use their arms.