Assist

Rescue is a group combat skill. While grouped with a friend, and both in
combat with an NPC, if you attempt to ‘rescue’ and succeed, the enemy’s
damage will all hit you rather than your friend. Especially useful for two
players where one is significantly better defended or more able to absorb
damage than the other (a tactic sometimes called ‘tanking’). Success of the
attempt depends on the skill rating as well as a comparison of your and the
enemy’s levels, dexterity, and speed.

See also: help skill rescue