Ah yes. The Walrein MT also works like this, and I use it. Here is how this attack works.
The attack does 50 damage (I think? the Walrein does 70), and has that effect. At this point, your opponent can choose to discard a card from his/her hand. If your opponent doesn't discard, or can't discard, the Defending Pokemon is now Paralyzed. If your opponent does discard, just do the damage.
This is applied each and every time Glaceon/Walrein attacks, meaning, each time you attack your opponent must discard a card or risk paralysis. It's quite an effective hand burner.