A clamping force mechanism and its method of use with electrosurgery allow a user to seal and/or join patient's particular vascular tissue; the mechanism is elongate with user and patient ends. An actuator is at the user end and the effectors are at the patient end. Each effector has a face of an area to contact the particular vascular tissue. A lost motion connection transfers user actuation to the effectors to hold a predetermined clamping force during electrosurgical tissue sealing. A yielding member in the loss motion connection clamps the particular tissue between the faces with a predetermined force. The yielding member is a spring, slip clutch or hydraulic coupling possibly near the actuator. An active electrode is carried on one end effector and a return electrode contacts the tissue so an electrosurgical energy supply connected thereacross delivers energy therebetween. A feedback circuit responds to parameters of energy delivered to tissue. A temperature sensor on one face and an impedance monitor respond to energy delivered. A control applies energy to held tissue to seal and/or join it. The method transfers user actuation of the effectors with the lost motion connection, maintains clamping force while sealing and applying electrosurgical energy to seal and/or join held tissue. Energy is applied to a temperature or an impedance. The method shifts the yielding member so the opposing faces clamp the particular tissue therebetween with the force from the spring via a transfer rod or by the hydraulic coupling or the slip clutch for a range of pressure.