Post-isometric relaxation is the brief window after a contraction when a muscle lets go more fully than before.
In 📝Resisted Soft-Tissue Release it is a working picture of why a short effort is followed by a pause and a chance to lengthen — not a lab fact about motor neurons and not a named certification. The window is used, not forced. If the muscle does not let go, the dose was too high or the person was still fighting.
Clients feel a give after they stop pushing. Chasing a bigger give by contracting harder is the miss.