A Bowler who bowls under their actual average to keep their league handicap high. Then when their team needs to win a game and/or series they bowl their actual average to increase their chances of winning.
Tom's actual average is 200 but during league play he bowls and averages 150. If the handicap for his league is to 200 and he averaged 200 he would have a "0" handicap. By averaging 150 he gets a 50 pin handicap. His handicap is based of the 150 average. So when his team needs more pins he bowls his actual average of 200 (or more)giving him and his team 50 extra pins (or more) making Tom a Sand bagger.