When the a/f ignites,not all of it is burnt. If the spark comes in too early,before the piston has a chance to compress it fully,you'll get the initial burn from the spark plug,which will heat the chamber. Then you'll get the second little "burn" from the hot a/f finally being compressed and self igniting(sort of a diesel like situation). You always want the spark before TDC because it takes time for the "burn" (flame front) to spread across the cylinder,but not too early. This is also the reason why timing is generally changed in regard to rpm(needs to be a hair earlier the faster the engine spins cause the a/f still burns at the same rate no matter what)
