How do active cards work? Can I only have 1 active card at a time?

[C O R P S E Y]

I think when I press Shift it gives me lifesteal. Somehow I have two other active cards equipped. How do I use them?

[Septimus Prime]

I think all activated active effects apply together when you Shift use them.
Like when i used Freeze + Almighty in active section they both applied together.
Passive effects below are active all the time though (and don't forget that if you click any ability then it shows what are the unlock conditions for each of them - like kill X amount of mobs with melee, use book charges X times etc. so you can start focusing and unlocking them faster) ... ofc we can have max 3 effects active at one time and no more.

[C O R P S E Y]

Oh nice thank you for the clarification

[Bankai9212]

Yeah, as mentioned above as you progress you'll learn how poorly implemented the system is with the 3 card equip limit.