Enforced mutual ignore is a last, desperate admin measure to stop long-standing personal conflicts continuing to disrupt threads.
This means that that those put on mutual ignore DO NOT respond, quote, comment or make any reference the other person in their posts, and continuing to do so will result in a warning, eventually leading to an automatic temp ban if they accrue enough warning posts.
It is very much a reluctant measure, but it is taken in the interests of the boards overall, and is only ever introduced after every other admin measure has failed to stop the personal digs and general unpleasantness.