A hyperspacial aggregation motor (or field engine) was a spacecraft propulsion device allowing for faster-than-light travel in hyperspace. The motor functioned by using engine fields to gain "traction" on and push against the energy grid.
Larger motors performed disproportionately better relative to smaller motors.
Motors were constructed of exotic material.
Fast spacecraft tended to have a larger proportion of their mass devoted to motors.
Care needed to be taken when engaging motors deep within a gravity well.
Intergalactic speeds were much slower than intragalactic speeds. The energy grid was more "tenuous" outside galaxies, making it more difficult for engine fields to gain traction.
A failed attempt to gain traction when "hitting the ground running" caused an energy backlash. In such a scenario, motors were designed to bleed off the energy by partially ablating; the ablation was visible to long-range sensors as an ablationary plume. The motors were disabled, and the spacecraft slowed. Without ablation the spacecraft would be seriously damaged or destroyed.