This was a spell Willow Rosenberg cast during her time as a dark witch. It granted her superhuman strength challenging that of a Slayer and possibly the impressive hand-to-hand fighting skills she demonstrated.
Da mihi vim.
As Willow materialized inside The Magic Box with Buffy and Dawn in tow, she finally found her targets: Jonathan Levinson and Andrew Wells. She tried to finish them off with blasts of mystical energies, but Anya was casting a protection spell, far from Willow's line of sight. The boys tried to run away, but to no avail, so they desperately held on to two swords conveniently found in their vicinity to try to defend themselves. Willow acknowledged the magics that was protecting them so she cast this spell. As she said the words, energy started to gather around her hands and electricity started to crackle from them. Soon enough, she was strong enough to flip a table away with one hand. However, her targets narrowly escaped with Xander and Dawn, which left her to deal with Buffy, who she was able to go toe-to-toe with with this spell. She was even able to knock a defenseless Anya out the moment she found her.