Uhhh, actually, this seems pretty balanced.
I mean, you say this card is affecting your hand which is a safe zone, but think about it. Not ONLY do you get to choose which card you discard, but if you can drop all of your cards fast enough, you don't have to discard anything at all. And it's not like these are difficult to kill. I think this is an excellent way to counter stall decks or OHKO decks, while giving a time rush deck a nice boost.
BEFORE you all accuse this of being OP, I ask that we see this card work in the trainer. Depending upon when you have to discard the card, this might actually be very easy to work around. If the discarding takes place at the end of your turn, then there's no way to be caught off guard by it. And even if it happens at the end of their turn, then the only time it can trick you is the turn it's played.
And Blood, who said your hand was safe? Last I checked, time was shoving expensive dragons in there and refusing to let you pay for them.