Harper's Afterburner is a trick to increase speed on a starship by injecting antiprotons into a ship's accelerator tanks by closing the Antiproton Solenoid Valve, pumping the tanks beyond capacity. Then reopening the valve and injecting the antiprotons straight into the core. It works for short bursts of speed, if done correctly. If not, then the fusion spillover destroys the ship. ("Vault of the Heavens")
Harpers terms of putting it: The sudden jump of mass pouring into our core under super pressure will give us a super burst of super speed.
It was done once on the Andromeda Ascendant with Harper running the night shift, racing a Nietzschean pilot named Agio. He completed the first few steps by closing the valve, fuelling the tanks and opening the valve wide open, giving the Andromeda a burst of speed, but the Antiproton Solenoid valve remained open when the ship transited to slipstream, causing the fusion spillover to go right back into the accelerator tanks, Andromeda contained the spill.