Biological tissue is sealed or fused to occlude an opening by compressing apposite sidewall portions of the tissue and applying sufficient energy to cause the fibers of the compressed apposed sidewall portions to intertwine and fuse with one another to form a permanent seal. The energy application is controlled by detecting a precursor fusion condition while applying the energy and before sufficient energy has been applied to achieve a permanent seal. The application of energy is terminated in a time-delayed relationship to the detection of the precursor fusion condition. The precursor fusion condition is detected from derivative values of the total accumulated amount of high-frequency energy conducted through the tissue.