The sail flapping because the sheet is eased too far or you are pointing too high. That small bubble along the leading edge is not a fault — it is a trim marker.
\"The sail is luffing\" gets said constantly on board and explained hardly ever. It means the cloth is flapping: the airflow has separated, the sail makes no drive, and the boat slows. Two causes — the sheet is eased too far, or you are pointing too high, in the extreme all the way into the no-go zone. That is exactly why stubborn luffing up is the classic pointing mistake: you sail a tighter angle but slower, and you drift sideways. Experienced sailors turn it around and use luffing as a tool. The rule of thumb: ease until a small bubble just forms along the leading edge, then sheet in a touch — that is the point where the sail is as open as it can be without losing power. And in a gust, easing decisively until the sail luffs is your emergency stop: pressure off, boat flat, control back.