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Common among inexperienced players is the act of "going south" after winning a big pot, which is to take a portion of your stake out of play, often as an attempt to hedge one's risk after a win. This is also known as "ratholing" and, while totally permissible in most other casino games, is not permitted in poker.
Table stakes are the rule in most cash poker games because it allows players with vastly different bankrolls a reasonable amount of protection when playing with one another. They are usually set in relation to the blinds. For example, in a $1/2 No Limit cash game, the minimum stake is often set at $40 while maximum stake is often set at $200, or 20 and 100 big blinds respectively.
This also requires some special rules to handle the case when a player is faced with a bet that he cannot call with his available stake.