Glenn,
you fly from la guardia to the hamptons and then back into the big apple.
your position now is (example) mid air, some miles east of la guardia and you prepare
for descend. Skyline popping up. follow me so far?
For future training purposes you save this very situation, before resuming flight.
So, what you have saved here, is a plane at mid alt at mid speed somewhere east of NYC, okay? This is where flight will continue next time you start x-plane and this saved situation.
Still not clear?
Ok. One day later you start x-plane and you chose that specific situation, let's call it "747 APP NYC.sit" to load. Once the situation has loaded, your airplane has already a speed of 300 knots and is heading towards the skyline of Manhattan. Agree?
While your flight is loading, and loading, and loading (your PC is mid aged and NYC scenery takes 10 minutes to load, you may have decided to leave the computer room and get a beer from the fridge. The kitchen is downstairs.
when you come back to your computer room you will see 300 dead passengers sprayed all over long island.
Clear?
Tell me if this is still unclear, i will be happy to make a movie.