Ignition timing is all about trying to make the most pressure, at the latest crank angle possible. Create the pressure too soon, and it simply tries to push the crank out the bottom of the block (no crank angle = no rotation). If you wait too long, the piston moves down the hole too fast, lowering the compression/pressure.

A motor at WOT will require the least amount of ignition timing at peak torque. This is because the engine is breathing best (highest Volumetric Efficiency %) at peak torque. With the cylinder packed with Air/Fuel at this RPM, it needs less timing. Think of it that the air/fuel molecules are packed closer together, and react quicker.

At any RPM before or after peak torque, more ignition timing is needed because there is less air/fuel packed into the cylinder (lower VE%). With the air/fuel molecules spread further apart, it takes longer to burn from one to another to the next. So since the burn time is slower, you need to ignite it earlier to achieve the same cylinder pressure.

Let say hypothetically that a motor makes peak torque at 6000 RPM. It's likely (somewhat) that it will want the same timing at 5,000rpm (1,000 RPM lower than peak torque), as it does at 7,000rpm (1,000 RPM above peak torque). Although you might think that because the piston has less time near TDC at 7,000 RPM vs 5,000 RPM, and might want more ignition timing. The reason it doesn't is because the higher engine speed actually rapidly mixes the air/fuel much better, which speeds up the burn process.

Just so you don't think I'm BSing you... lol "As engine speed increases, the intensity of turbulence, swirl, squish, and tumble all increase, resulting in a faster flame speed." (Engineering Fundamentals of the Internal Combustion Engine, p280)

Basically the flame speed increases directly with RPM, which offsets the need to start the burn sooner.