1. A method for performing electrosurgical tissue fusion comprising the steps of: closing a pair of jaw members of an electrosurgical instrument about tissue; applying electrosurgical energy at an output level to tissue from an electrosurgical generator; determining whether tissue reaction has occurred as a function of a minimum impedance value and a predetermined rise in impedance, wherein tissue reaction corresponds to a boiling point of tissue fluid; generating a target impedance trajectory as a function of measured impedance and desired rate of change based on the tissue reaction determination, the target impedance trajectory including a plurality of target impedance values; interrupting the application of electrosurgical energy to tissue; cooling the tissue; and releasing the jaw members of the electrosurgical instrument from tissue.