This is a seat post stand to stabilize the bike when folded.  It slips on easily and is secure.  Made from UV resistant plastic.  You may make a mark on the seat post to repeadedly set the desired folded seat position. 
 
Available in three angles:
14 degree - maximum compact folded seat configuration - good for no rear seat bag - seat post flush with frame
 
11 degree - compact folded seat configuration - good with rear seat bag - seat post raised 1.5 inch
 
8 degree - folded seat configuration - more room for rear seat bag clearance - seat post raised 2.4 inch