The song is progressing normally, and then something happens that pushes it to another place entirely, taking your brain along with it for the ride...
E.g. the drums kicking in in "The Air Tonight" .
My two favourites:
1) Adam and the Ants - the guitar riff straight after "...too many antics in the forbidden zowowowowowowowone..." in "Ants Invasion"
2) Janes Addiction - the bit in "Three Days" around "we saw...... shadows of the morning light"