In Castlevania: Circle of the Moon, you pick up these cards that are used in something called the "DSS System" do give you special abilities, you have to pick one from the top row and one from the bottom. And then you press the L button to activate the combo.

For example: choose the Mars and Manticore cards and your whip will be replaced with "Poison Claws"

Then you press L again to deactivate it.

What's interesting is if you press L to activate a combo

And while your character is going through the animation of activating the skill, you go to the status screen

And pick a combination of cards that you don't actually have, you get its effect anyway! Like here, this combo activates a familiar that follows me around

The only drawback is that some of the effects are kind of hard to figure out what they do, and if you do manage to figure them out, the game won't actually record the description for you (unless you eventually collect the cards, of course).

One of my favorites is this combination

Which gives you the Item Crush ability. It costs 20 hearts, but you unleash a super attack with whatever sub-weapon you have equipped, if you put in the correct button combo

It's quite useful, especially on bosses and powerful normal enemies.