The tan sao is described as a palm up hand. It is an attacking hand oriented forward.
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The laan sao will be described as locking arm. It is used to free some space from the opponent in front of the body, to build up space in which...
As kwan sao, we describe a move composed of a tan sao and a deep bong sao. It is a defensive as well as an offensive move.
Wu sao will be described as protecting hand. It is the last line of defense in front of the body, before the attack reaches it.