Think Jeff McBride not the Buck Twins.
Essentially if you imagine splitting the face and back off a card, and then gluing those two pieces of paper together, that's a manipulation card. Very thin and flexible to allow a ton of cards to be palmed or back palmed more easily. This allows for things like producing multiple split fans in a row and other such classic manipulation routines.
In theory you can turn a 'normal' deck of cards into manipulation cards by running each card over the edge of a desk or such several times, and then putting the lot into a card vice and clamping it down super hard. I don't know if that's actually effective, though.