I've got something similar, if you buy one of those UV protection tops the surfers wear they do exactly the same thing but cheaper
.
Also if you wear it right under your jersey, mine slips really easily under the jersey fabric & makes sliding easier.
as Dave said they take the sweat away from your skin or something, and keep you cool.
It's not neoprene, it's sorta like lycra
. Won't help with bounces at all, I don't think.
edit: similar to this, sorta.
http://cgi.ebay.co.uk/BODYGLOVE-RASH-VEST-Thermolator-wetsuit-surfing-surf_W0QQitemZ7241359132QQcategoryZ22712QQrdZ1QQcmdZViewItem
But anyways the dye balance top is designed specifically for keeping you cool, don't go for cheap alternatives kids.