The release decides curl and consistency: handle from 10 or 2 o'clock to just before 12, arm extension, counting rotations and follow-through, step by step.
The honest answer: the release is the moment the stone leaves your hand, and it decides whether the whole clean delivery before it was worth anything. Curling Canada calls it "the moment of truth": a poor release can cancel out everything that went well before [1]. It begins with the arm extension plus the turn and ends with the follow-through (the carry-through of the arm) [1].
What exactly happens? The handle sits at 10 or 2 o'clock during the slide and is turned to just before 12 o'clock, where it separates from the hand [1]. This small turn is why the stone runs a curve. The rotation also makes the stone run more predictably [4]. At the same time the stone must be clearly released before the hog line, otherwise it is removed from play immediately [3].
How hard is it? The release is one of the finest parts of technique, because you cannot see it, you can only feel it. Expect many stones and feedback from outside. There is no time promise.
Set the handle at 10 or 2 o'clock and keep that position from set-up into the slide [1]. Hold loosely with the second finger joints [2]. A handle that is twisted at the start cannot be saved by the release.
Milestone: three slides in a row where neither handle nor grip pressure changes on the way.
Extend the throwing arm smoothly over at least six feet (about 1.8 metres), longer on heavier throws [1]. Practise this first without a turn: the stone should leave straight.
Milestone: the straight-thrown stone stays on the line to the broom.
Start the turn about halfway through the arm extension, with an even tempo on both turns [1]. The handle travels from 10 or 2 o'clock to just before 12 [1]. Do not turn abruptly, turn evenly. Cadence counts.
Milestone: the stone visibly rotates without "flipping" and without wandering [2].
For beginners on club ice, club materials mention about 2 to 3 rotations over the length of the sheet [2][5]. Advanced curlers use 4 to 5 according to Curling Canada (up to 5.5 on strongly curling ice), because more rotation needs less ice and keeps the stone tighter [1]. Start small, count along and add only when the rest is solid.
Milestone: three stones in a row with roughly the same number of revolutions.
Hold the arm position for at least six feet after release [1]. Your check: on an in-turn you see one to two knuckles and no palm, on an out-turn one to two palm pads and no knuckles [1]. Finish the movement with the hand in a "handshake" position [2].
Do not turn to "bend" the stone, turn to make it predictable. The authors of an ice-surface study note that the rotation steers the stone sideways and lets it run in a more predictable way [4]. Exactly how the curl arises is still disputed in research [6]. For you that means: rely less on theory and more on an even rotation, time after time.
For a right-hander, the in-turn rotates clockwise and the out-turn counter-clockwise; for a left-hander it is reversed [3]. You also see it in the hand: after release you see knuckles on an in-turn and palm pads on an out-turn [1].
Give the handle a small turn at release, from 10 or 2 o'clock to just before 12 [1]. Without rotation the stone runs straight. A study found that the rotation steers it sideways and makes it more predictable [4]. Release loosely before the stone reaches the hog line [3].
The mechanism is still not conclusively settled. One study supports the "scratch-guiding" theory: the stone leaves scratches in the ice that steer it sideways [4]. A review mentions controversy about the underlying physics [6].
The Freeze in Curling: Stopping Right Against Another Stone