Sono_hito said:

My guess would be electric muscle stimulation pads. Force you to flex against it.

Legati said:

Oh, that's... That's a really hawt idea.

This. Plus, it also stimulates sensory nerves, so I can imagine that the slectro-stim itself would bring her pleasure. If you look, the clip electrode on her clit has two wires coming out of it. It could be that each side of the clip is one pole, or one half lf the circuit, but that is highly unlikely because of how difficult those would be to manufacture, and thus how much cheaper it would be to just use two separate clips. A more plausible explanation would be that both wires are connected to the same spot, and the whole clip is an electrode. In that case, it is one half of two separate circuits, and the electrode pads on her ass are the other halves.

TL;DR: The current goes through her body from the electrodes on her butt to the clip on her clit, and vice versa, making her clench and also providing additional stimulation and pleasure. I haven't had the luxury of trying it on someone myself, but I have heard it is quite nice for those who like it.