The peel clears a guard with hard weight, and your own stone leaves play too. When it makes sense, free-guard-zone rules, technique and safety.
The honest answer: a peel is a shot designed to remove a guard, and your own stone leaves play too. The world federation's glossary keeps it short: "a shot designed to remove a guard" [1]. A media glossary extends it to "guard or guards" [2]. If you already know a takeout as "hard": the peel is the heavier, more defensive variant where it's not about where you lie but about taking away the opponent's cover.
Whether the peel is right depends on the game situation. The US federation material describes a conservative style with few guards and many takeouts, when you don't want to score a big end but want to blank it, score one or even give one up [3]. That fits the peel.
How hard is it? You need a reliable takeout weight and a good line. There is no timeline.
Before the delivery of the sixth stone of an end, a delivered stone may not remove an opposition stone in the free guard zone from play. If it does, your stone is removed and the displaced stones are replaced [1]. For a peel on an opposing guard, that means: only from the sixth stone of an end. The rule does not apply to your own guard [1].
Milestone: You can explain to a teammate when a peel on an opposing guard is allowed.
For hard hits and peels, Curling Canada says the step with which you generate weight is important [4]. For heavy throws you start the push later [4]. Elite curlers showed faster and shorter push-off phases in takeouts than in draws [5].
Milestone: Your takeout hits the target stone with clean speed in three of five cases (practice measure).
You don't want to brush the guard but hit it centrally so that it flies out. Your stone may fly beyond the target spot, unlike in a hit and stay. What matters is that no stone in the house is endangered (practice).
The US material warns: rarely peel a "double corner guard" (two guards in the corners) [3]. It also says: don't do the opponent's work. A hit at the front of the house can push their stone against yours and tip the end [3].
If two stones lie frozen together in the FGZ after a peel, the back one can be dragged along when the front one is hit. A coach's essay calls this the "drag effect" [6]. It is not a research paper.
After a hard hit, stones fly in all directions. Your team should be in position, not in the way.
The peel doesn't have to be the hardest throw of your life: in elite curlers, lateral sway in takeouts was lower than in draws, and the higher the push-off speed, the more stable the sideways position (r = -0.46) [5]. A decisive, well-timed push is more stable than a half-hearted one. Practise it with a coach, though, not alone.
A peel is a shot designed to remove a guard [1]. Your own stone also leaves the sheet. A media glossary describes it as a shot that clears one or more guards [2].
Only when the sixth stone of an end is played. Before that, your stone is removed if it removes an opposition stone in the free guard zone from play [1].
A reliable takeout weight: according to Curling Canada, for hard hits and peels the step with which you generate weight is important, and the push comes later [4]. Practise with a coach.
US material mentions it in a conservative style with few guards and many takeouts, when you don't want to score a big end [3]. Whoever is ahead keeps the game clean [3]. The skip decides.
The Double Takeout in Curling: One Stone, Two Opponents Out