The exchange happens on resolution of Arcanum Things’s Equipment swap ability and is simultaneous.

As Equipment swap resolves, you may reveal an Equipment card from your hand. If you don’t reveal an Equipment card, or if the exchange can’t be completed (such as if Arcanum Things is no longer on the battlefield, you don’t own Arcanum Things, or the new Equipment can’t be attached to the creature Arcanum Things is attached to), nothing happens.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Playtest cards use a modified version of game symbols, such as {T} and {W}. These modified symbols should be treated as the standard symbols during play.

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.