Take a kid on a swing, help to push the kid on the swing. When he comes back to you you try to get the hands on the swing but you always have to push the swing after the dead point to speed up. If you push it directly at the dead point you need a lot of force to speed up the swing but if you start to push it before the dead point you can actually slow it down.

In our engines it is the same. There is a small delay beween ignition (get the hands on the swing) and combustion (the push of the swing) That delay is caused by several things:
- cool down of the plug wire (the colder the wire gets after the combustion the next ignition will be less stong)
- flex on the internal parts (more rpm -> higher moving piston -> more compression)
- the fuel itself (the combustion speed is depending the oil)

The first 2 are rpm related and is indeed a wanted factor to get an advanced timing. The last one is just something to take care off when switching fuels and you still want the max performance.

The advanced timing needs to shift in a same amount of the rpm but sadly that is not always controlable so with a high compression setting you can get more motor performance on the low and mid range but on the high rpm range the ignition is to soon slowing down the piston and with that giving huge forces on the crank pin and rod.