Locking BTC against a price move
Bitcoin’s price moves. Locking some of it until a date is useful: you get paid now for agreeing to a limit on that move.
Cap gains
You lock BTC until a date. You are paid now. If BTC stays under your line, you get the BTC back. If it goes above, you give up the gains above that line.
Take the loss
You lock BTC until a date. You are paid now. If BTC stays above your line, you get the BTC back. If it falls below, that loss comes out of what you locked.
Cap gains is selling a call. Take the loss is selling a put.
Why the put is different here
Settlement pays Bitcoin, not dollars. A cash-secured put sets dollars aside to pay the dollar drop. This only locks BTC, and that BTC is worth fewer dollars as the price falls, so it cannot promise an unlimited dollar loss. The instrument is a limited put: you cover the fall down to half your line. At half the line or lower, the desk keeps what you locked, and you keep only the payment you already received.
Three steps
You send in the BTC you are locking.
You are paid now, and the locked BTC is funded into the vault until the date.
On the date, the vault is split by where the price finished.