Re: Dust Runner Engineering
FYI: If you've ever seen the Jodie Foster movie, Contact, I picture the inside of a J-drive engine looking something like the machine they build. In technical jargon, it's a precessing magnetoscopic gyro... but the engineer's slang term is 'spindizzy'.
When it's spinning at the extremely high speeds needed to do a hyperspace jump, it is easily affected by strong gravity. Safety measures shut it down instantly when that happens, or else the imbalance would tear the engine apart.
Thus a gravity field produces a noticeable 'wobble' in one of the spinning loops. If the gravity is only temporary, it'll self-stabilize again, but the effect has still been interrupted, which drops you out of J-space.
It's all right there in the book Shamus is reading! :-D