found this here
http://www.fagan.co.za/Bikes/Csuck/
The chain fails to disengage from the bottom teeth of a front chain-ring ; instead the teeth snag the chain and carry it up and around the rear circumference of the ring, winding it back onto itself, and jamming it between the chain-rings and chain-stay.
Because it usually occurs unexpectedly during forceful pedalling, the chain can be permanently twisted, teeth can be damaged, and chain-stays of aluminium or carbon-fibre can also be damaged by the chain being wedged hard against them.
It occurs most frequently with small chain-rings (granny-rings) of triple-ring MOUNTAIN BIKES, but does also occur with larger rings, and on ROAD BIKES. There are two main forms of chain suck :
1-ring suck : the chain is snagged by teeth on a single ring during normal
pedalling (usually the granny-ring)
2-ring suck : the chain snags on teeth across two rings when changing
gears (usually from the middle ring to the granny)
the link goes into more detail.