Simple (mostly found on basic thumbplate systems and vintage instruments): http://www.howarth.uk.com/pic.aspx?pic=./wo/HowarthS10.jpg&pid=35101
Both 8ve keys aren't linked, so you have to release the thumb 8ve key at the same time you open the 2nd 8ve key (and the other way round when going back down).
Semi-auto (the most popular, found on both student and pro models): http://www.howarth.uk.com/pic.aspx?pic=./wo/HowarthXLObBWCon.jpg&pid=35118
The thumb can remain on the back 8ve key while the 2nd is opened, and the lower 8ve vent closes automatically when the 2nd 8ve is opened, and opens when the 2nd 8ve key is released.