BTC 5-minute markets (Up/Down) — how do they work?

BTC 5-minute markets are fast markets on Bitcoin's price move:

• Each window lasts 5 minutes. The opening BTC price is locked at the start.
• UP wins if the price is higher at the end of the window; DOWN if it's lower.
• The market resolves automatically at the window's end using the BTC price.
• If the end price equals the open, stakes are refunded (void).

It's a separate format: split / merge / redeem operations aren't available for it.