After switching WP while using a follower, he/she regains all of his/her HP if she is dead (full revive, basically). I don't know yet what this could be used for, besides the obvious "don't have to wait 30 seconds between follower resurrections".
Their cooldowns don't seem to reset, sadly, but still their HP does. If you think this is intended, just delete this thread. If it wasn't, then...whatever, just leave it here.