An apparatus for modulating flow in biological conduits has a valve mechanism which has an open state and a closed state. The valve is applied and secured to a particular biological conduit while being maintained in an open state. An actuating mechanism is provided which actuates a change of state of the valve from an open state to a closed state in response to leakage from the biological conduit. In one embodiment, actuation is automatic and in another it is manual but remote. A preferred embodiment is a pinch valve such as that used in temporary micro vascular surgery to which is added an effectively resilient element interposed between opposing members of said clip. The interposed element is configured such that it is unable to maintain the clip in an open position unless the clip opening is augmented by manual or other means, and it is able to maintain the clip in an open position if a blood vessel is placed within the clip and has an adequate flow. Also in a preferred embodiment, the interposed element is ratcheted such that the clip is more readily changed from an open state to a closed state than it is from a closed state to an open state. A preferred embodiment for remote manual activation requires the resilient element to be partially ferromagnetic such that a manually activated magnetic field is sufficient to cause the leaf spring to trigger transition of the clip to a closed state.