Slowing was added during the Beta 3.4 update as a replacement for Sharing to fit more with the rework of the Patience. It puts a paralyzed enemy monster back into its owner's deck when used and it will cost 3G more (than it's base cost) to place it again. Because of this it's suggested that people keep 2 or 3 of this card in their decks and save them for later in game when the opponent places more expensive cards on the field.
Keep in mind that it cannot be used on your own monsters. (And even if it did, it would cost 3 more to place it again, and it would be put into your deck. You're way better off with Doggo, who puts it in your hand and can be placed again for the base cost.)