One sentence, and the conditions on it are where the detachment lives.
Once per turn, per unit, when a friendly THOUSAND SONS PSYKER unit (excluding MONSTER units) successfully manifests a Ritual, that unit heals D3 wounds.
Read each clause precisely:
- It triggers on successfully manifesting, not on attempting. A failed Psychic test heals nothing. This matters more than it sounds, because it changes how you pick which Ritual to attempt with which model: the cheap Rituals are not just easier to land, they are the reliable healing.
- It heals the manifesting unit, not a unit of your choice. You cannot direct the heal to whatever is hurt. You have to cast with the unit you want healed, which is a positioning constraint rather than a free bonus.
- MONSTER units are excluded. Magnus the Red does not heal himself with this.
- Once per turn per unit, so a unit with two casters in it does not double up.
What that adds up to is a rule that rewards spreading your Psykers across several units rather than stacking them. Four separate Psyker units each manifesting one Ritual heals four units for D3 each. One unit containing four Psykers manifests up to four Rituals and heals once.
And because there are only four Rituals in the whole army rule, four is your ceiling on heals per turn regardless of how many casters you own.