Well, sometimes it happens that someones mind is disrupted by very high mana depletion or by channeling many hitpoints to mana. Those people are weak because their mind is disrupted. Druids - the masters of harmony - and Priests - the healing specialists - know one way how to renew harmony in tired minds of their friends. It is impossible to calm mind of your friend when either of you is fighting. Also, any combat in the same room noticeably reduces effect of this spell, since combat is such a distraction.