Bell pits are simply bell shaped pits. A shallow shaft is sunk to the seam and then the seam is mined until the air becomes foul, the pit is then abandoned and another opened a little further along the seam. The name comes from the profile of the pit.
These are a good example.
http://viewfinder.english-heritage.org.uk/search/reference.asp?index=25&main_query=&theme=ARCHAEOLOGY&period=&county=&district=&place_name=&imageUID=82666&=&JS=True