where the chain gets dragged up between the chainset and the chain stays, jamming itself in.

Usually happens when you're changing down onto the granny ring (small). Can be nasty as it can score the frame which, over time, can cause hairline cracks.

Trick is to make sure your front mech is correctly adjusted and if it does happen, stop your pedalling force immediately.

check out www.wiggle.co.uk for good new prices on WTB Speed V and Rocket V seats (http://www.wiggle.co.uk/v2_product_list.asp?CategoryName=Saddles - Performance&Cat=)

also they come up on Ebay quite a lot

(http://search.ebay.co.uk/search/search.dll?query=wtb&catref=C3&socolumnlayout=3&lotr=1&from=R2&sacategory=16169&BasicSearch=)