Q: Dear Sage
What exactly does the second effect of protection from evil do, anyway?
--Too many questioners to list
A: The Sage feels your pain. While the first and third effects of protection from evil are relatively straightforward, the second is less clear.
The key phrase that defines this particular effect of the spell is as follows: “…the barrier blocks any attempt to… exercise mental control over the creature (including enchantment (charm) effects and enchantment (compulsion) effects that grant the caster ongoing control over the subject…).”
(The spell also blocks attempts to possess the creature, but effects that accomplish this are so few as to barely be worth mentioning.)
The first part of this phrase describes the basic criteria by which the DM should judge protection from evil’s effect: If the incoming effect attempts to exercise mental control over the creature, protection from evil likely suppresses that effect.
The parenthetical portion of the phrase provides two specific examples (pointed, obviously, at rules elements of the Player’s Handbook) to help judge what exactly is meant by that:
Enchantment (charm) effects. Simple enough--protection from evil automatically suppresses any enchantment (charm) effect, such as charm person or enthrall.
Enchantment (compulsion) effects that grant the caster ongoing control over the subject. This is where adjudication gets trickier, because you have to decided what “ongoing control” means. The Sage recommends a broad definition, which includes any non-instantaneous effect that prevents the target from exercising full control over its own actions.
Examples would include the obvious (such as command or dominate person), but also the less obvious, such as daze, sleep, and Tasha’s hideous laughter. Such effects would be suppressed for as long as protection from evil lasts on the target.
There are still plenty of enchantment (compulsion) effects that don’t grant the caster ongoing control over the subject. Heroism, crushing despair, mind fog, power word blind, rage, and touch of idiocy are examples. Protection from evil has no effect on such spells.
But what about mental control effects that aren’t enchantment effects, such as psionics? In such cases, the DM must use the rules and his own best judgment in concert to adjudicate the effect. Psionic powers of the telepathy discipline are the equivalent of enchantment spells, for example, and thus are affected in the same way. Non-spell effects that closely mimic enchantment spells should be treated as if they were spells of the appropriate subschool (charm or compulsion).
