Quote Originally Posted by Motu
Why don't people reply if they do know? - it's entertaining?
But I mean Ixion already had explained what it is.

A quick speel:
The rings don’t rely on tension to seal the cylinder (well at anything more than kickover speed). The top ring is pushed out by the pressure of the charge contained above it in the readily shrinking volume. The 2nd ring seals what the top ring lets by. The ring expander is (this is a guess here is likely to help the 2nd ring move around a bit to stop it being so prone to gumming up & make sure it is 'out' as the engine starts which would aid compression at starting speed.

Some manufacturers use them, some don’t. Largely it seems on lower speed engines, Although I think my RZ had them.