On a 2 stroke glow, the only way to set the "timing" (when the detonation occurs) is by compression ratio, nitro content, glow plug heat, engine load, pipe length, atmospheric conditions, or a combination of the above.

Being able to adjust the compression ratio will help you get the optimum power from your engine.

Lowering the compression ratio will retard when detonation occurs, the same as lowering nitro, reducing load, colder plug, or longer pipe.

The main advantage to lowering the compression is to eleminate pre-detonation..where the charge fires prior to the optimum for the nitro/load/plug...all that. If the engine is pre-detonating, it is working against itself and causes a loss of power, added wear on internal components, extra heat, burned glow plugs, etc.

A far easier method than adding a head shim is to reduce nitro or run a prop that doesn't load the engine as much.

This is just a broad overview of compression..I'm positive you will get a much more detailed explaination shortly.