If a model who is wearing purity seals falls back, roll one extra D6 for its fall back distance, and then pick the D6 results you want in order to determine the distance fallen back. If a model with purity seals is part of a unit then this ability applies to the whole unit, not just to the model with the purity seals.
Purity Seals
Source: Codex: Space Marines, p. 34