Mind Heal is one of the Mystical abilities. You need 20 Mystical use points to be able to cast it. If you have 19 use points plus your bonus 1 pt for being a Mystical user, you can still cast Mind Heal even though it won't show up on your Mystical Skills menu. This is true for every Mystical Skill ability that costs multiple points. Purpose
Mind Heal restores your Hit Points to full. Strategic use
Mind Heal is primarily desired to aid in killing The Red Dragon, though once a player has at least 20 Mystical Skill points, it's always an option for offline player fights. Usually, other Mystical Skill uses take priority: Light Shield and Shatter in particular.
When a player achieves 32 Mystical Skill points, the possibility of Lightshield + Mind Heal opens up in Red Dragon fights. Mind Heal's effect is similar to what a fairy will do for you. The only difference is that with a fairy, you are automatically refreshed when you lose all your Hit Points ; with Mind Heal, you must judge for yourself when to cast the spell--and waiting too long will get you killed. Most players wait until they have only a few hundred Hit Points left, hoping to take as many dragon hits as possible before needing Mind Heal. With Lightshield on, the dragon's attacks are all halved, so any strike that isn't Flaming Breath will probably suit your purposes. It's always a gamble, though. Having a fairy as well makes a dragon kill reasonably assured, since if you wait too long, it will kick in, and if you don't, you still have it against the possibility of a Flame Breath.
If you have extra HP left over from Jennie HOTT, it will erase them.