In martial arts, this is the defensive act of wrapping your arm (or leg, if you are especially flexible) around the attacking arm of the opponent. This is usually followed by a joint lock, throw, or a quick counter-attack in a manner that would leave few openings for further attacks by the opponent. If, for example, an attacker threw a right hook at you, you would first block with your left forearm pointed up, then pivot your wrist and hand outward and underneath the attacking arm. Now that you have their right arm entangled in your left you can pull them off balance. This maneuver can be done with less effort by simultaneously attacking them with a palm strike or eye gouge with your right arm.
It should be noted that action should be taken as soon as the entanglement is executed, or else the opponent might take advantage of the situation and throw you off balance. This is especially true for larger and stronger opponents.
Note that I take no responsibility for the usage of this tactic in any circumstance.