Interviewer: What can you tell me about how a turbine engine operates?
Me: Uhhh...suck, squeeze, bang, blow?
FWIW turbines and piston engines operate on the same principles. The drawback of piston engines, if you want to call it that, is they lack continuous ignition and lose efficency converting vertical acceleration to a rotary movement.
When I was hired by my airline my response about turbine engine theory was pretty much what I wrote. I was flying Chieftains at the time of the interview. While I can't speak for interviewers at any airline I think most of them are merely looking for familiarity with things that you should know based on your experience. If you fly a piston twin you should be able to explain how any system of the aircraft operates and how the backup systems function. If you work for a 135 operation you should be able to answer any question on Ops Specs. For instance can you accept a contact approach? When can you cancel IFR? When is a takeoff alternate required?