Sleights are a combanation of at the most 3 cards. To add a card to a sleight you want to use, you press "R" and "L" at the same time during battle on the card you want to add.
Depending on the card type, number, character, or weapon determines the sleight that you will use when you press "R" and "L" together again once you have three cards or when you're on the reload gray card area.
Sleights can still be beaten by cards that are higher than the total number that the 1-3 cards add up to. (And 0) While sleights are useful during battle, you lose cards quicker the more you use them.
and items cards don't make good sleights