What makes a motor a short stack? How much shorter is it than regular? What does it do? Or is it just a gimmick....

ROAR recently changed their rules for stator lamination "stack" length:

Original language (8.5.4.2): "The overall stator length parallel to the motor shaft shall be a maximum 21.0 mm"

New language: "The overall stator length parallel to the motor shaft shall be minimum length 19.3mm, maximum 21.0mm"

A normal stator might be in the range of 20mm for a 540-style brushless motor. A short stack will be close to the 19.3mm limit.

Making the stack shorter is one way to reduce the resistance of the windings. (Another way is to use larger wire, but ROAR rules already specify a maximum diameter for each motor type.) Lower resistance gives more current, and hence more power.

The difference might be only a few percent, but that's a useful amount in restricted classes.

Tech team test.

not a gimmick, the fast motors right now are all short stack

The d4 17.5 short stack is also ~10g lighter than the d4 17.5 non-short stack.