Here's my go at a FAQ:
Trashing is not optional. If you trash a card costing
, or if you have no cards left in hand to trash, nothing else happens. If you trash a card costing
or more, you gain a cheaper card and a Gold. Both come from the Supply, and are put into your discard pile. The cheaper card goes into your discard pile first. For example, if you trash a Silver (costing
), you could gain an Estate (costing
). There will almost always be a cheaper card in the Supply, since Copper and Curse cost
, but if there aren't any, you still gain a Gold. If there aren't any Gold left in the Supply, you still gain the cheaper card. Cards that cost only
(like Transmute from Alchemy) or only
(like Engineer from Empires) do not cost
or more.