Definition
An inside-out (IO) throw is released so that the disc starts on the body-side of the Mark and curves out to the receiver. The "inside" is the angle at release; the "out" is where the disc ends up. It is the natural shape for breaking a force at short-to-medium range, because the release point is past the marker's hands before the disc has flattened out.
In Context
The IO is one of the two break-side throw shapes — the other being the roll-curve (or outside-in). Both serve the same purpose: putting the disc on the Break Side without the marker getting a hand on it. The IO is generally the more reliable shape for shorter break throws, because the curve is tighter and the timing is forgiving.
The IO is built around a Pivot — the lead foot steps inside, the release angle is set, the disc leaves on the curve. Without the pivot, the IO is a wrist-flick; with it, the throw is a deliberate decision the marker cannot defend.
See Break Mark 1 for the foundational drill and 70s Throwing for the standard warm-up that keeps the IO release dialled in across a season.