S basicaly will hold it in 3rd, L will hold it in 2nd
D uses 1,2,3,& 4
S uses 1,2 & 3
L uses 1 & 2
useful when 'getting a move on' - for example, slowing for a roundabout - stick in s, then possibly L to keep the revs up, then boot it out of the roundabout..
or for engine braking on long hills...