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Constraint power transfer is redistribution of energy between parts of a mechanically connected system through constraint forces. An ideal connection may exert nonzero power on each body while exerting zero total power on the combination. Power on a body is the actual force–velocity scalar product; the zero-virtual work property of an ideal mechanical constraint concerns the sum over admissible virtual displacements.^[ref]

An original ideal rigid link calculation makes the cancellation explicit. Let two endpoint positions be ${\mathbf{r}}_A$ and ${\mathbf{r}}_B$, joined by a massless link of fixed length $\ell$. SetFor axial tension $T$, the link exerts $+T\mathbf{n}$ on A and $-T\mathbf{n}$ on B. The powers supplied to the endpoints areThe endpoint velocities need not be identical: the link can rotate. Only their components along the link must agree.

For stipulated $T=12\mathrm{N}$ and common axial endpoint speed $0.40m/s$, the link supplies $+4.8\mathrm{W}$ to A and $-4.8\mathrm{W}$ to B. It transfers energy without supplying net energy to their combined system. A load cell measuring axial force and tracked endpoint positions measuring velocities would permit these two powers to be compared. External forces determine how the endpoints subsequently accelerate.

Equal and opposite forces alone do not prove power cancellation. If the connector stretches, $\dot{\ell }=\mathbf{n}\cdot ({\mathbf{v}}_B-{\mathbf{v}}_A)$ and the same axial-force calculation givesAn elastic connector can store the corresponding elastic energy; a dissipative connector can convert mechanical energy into heat. Likewise, an externally prescribed moving support can deliver nonzero net constraint power to a subsystem. The location of the system boundary determines whether a measured exchange is internal transfer or externally supplied work.