Isometric is a contraction in which the muscle works without the joint moving, the kind used in 📝Resisted Soft-Tissue Release.
The limb stays put while the person pushes. It is a generic contraction type, not a gym program and not a medical strengthening protocol. The work is brief; then the person eases off. Joint movement during the isometric means the effort or the contact needs a reset.
Clients feel work without going anywhere. That is the point.