A question about 2x drop.

If I get Pianus down to an inch of health, then buy a 2x drop card and go back in and kill it, will the drops be doubled? Or does it have to be in possession the majority of the time for it to have an effect?

I've done this many times and it doesn't seem like Pianus' that I do this with drop enough, while Pianus' that I kill with the card in my inventory the entire time noticeably drop a few more items.