Fusion overburn

Two shuttlepods perform a fusion overburn

A fusion overburn was a procedure, considered quite dangerous, in which an impulse engine's power output was pushed past the normal maximum safety level in an attempt to generate a great amount of thrust.

In 2153, Sim, a clone of Commander Tucker, came up with the idea of using a fusion overburn on two shuttlepods, connected to Enterprise NX-01 via grappler cables, to tow Enterprise out of a polaric field in which it had become trapped. Although the idea was met with a certain amount of skepticism, the plan eventually worked, and Enterprise escaped. (ENT: "Similitude")

During the procedure, according to the Star Trek Encyclopedia (4th ed., vol. 1, p. 288), hydrogen was injected into the exhaust plume to increase thrust.
In the final draft script of ENT: "Similitude", fusion overburn was described as involving ejection of plasma flares from engines during the procedure.