[irvingj]

It's a way to temporarily (a few seconds at best) bypass the normal fuel delivery system. One "shot" of spray will give the cylinder(s) enough atomized fuel to ignite and for the engine to run briefly.

It's simply a method of determining if the problem is in the fuel delivery system or not. If it fires up after spraying (but not before), it pretty well means the engine's NOT getting any fuel through the normal system.