Two clauses, and the first is the detachment.
Each time a THOUSAND SONS VEHICLE unit from your army is selected to shoot or fight, apply one of the following:
- If that VEHICLE unit is within 6 inches of one or more friendly THOUSAND SONS PSYKER models, you can re-roll one Hit roll, one Wound roll AND one Damage roll.
- Otherwise, you can re-roll one Hit roll, one Wound roll OR one Damage roll.
That is the whole detachment in one comparison. Near a Psyker you get three re-rolls. Away from one you get one. Not one third better: three times as many.
And the three you get are the three that matter, because they are the three stages a single attack has to survive. Being able to fix a failed Hit, then a failed Wound, then a low Damage roll on the same activation means you can convert your best shot into a landed one from end to end. On a vehicle with a single high-damage weapon, that is close to guaranteeing that the shot you cared about does its job.
So the practical instruction is blunt: your Sorcerers walk with your tanks. Not behind them, not on the other flank. Within 6 inches, every turn, or you are playing a materially weaker detachment than the one you paid 2 DP for.
The second clause is a nice piece of thematic design. Each time a THOUSAND SONS VEHICLE model with the Deadly Demise ability is destroyed while within 6 inches of one or more friendly THOUSAND SONS PSYKER models, its Deadly Demise inflicts mortal wounds on a 5+ instead of only a 6.
That is the same 6-inch condition again, and it means your dying tanks explode more reliably. It also means the positioning discipline pays out even when you lose the vehicle, which slightly softens the risk of pushing war engines forward. Note that it only applies to models that already have Deadly Demise, so check your datasheets.