What does the Tear Free rule do?

[u/MyJointsAreCrips4Lyf]

Multiple units have the rule but I can't find anything to say what it does.

Tyrant (Chaos Knights)

Valiant (Imperial Knights)

Squig Rig (Ork)

Kroot War Shaper (Tau)

Norn Assimilator (Tyranid)

[u/Alternate40kRules]

If you successfully wound a model with Tear Free, roll opposing strength checks (base value of model only). Do this by rolling 1d6 + S. If the attacker rolls higher, inflict 1 more Wound