It is also called the compression bump technique.
You prime the engine, and flip a couple of times without glow. When the engine feels good and ready, add glow, grip the prop real tight, and try to turn it over the compression. When it knocks back, it is an indication that it is ready to go. Now lightly flip the engine backwards against the normal rotation, just bump it against compression, so when it kicks back, it starts running in the right direction. Bij the time that happens, have your fingers out of the way! A slight twist on the spinner also has the same effect. This works on all glow engines, from 1/2A to 2.50. For the larger engines, it is the only way!