Okay so I'm running at an angle to you - facing you all of the time, but you can shoot me in the back.

Not that.

Okay then I'm running at an angle to you, and turn to face you as you shoot and you can shoot me in the back.

Nope not that either.

Ah, while I'm running at an angle to you, before I turn, you shoot, then I turn to face you and fall over with a bullet in my back.

So that would mean that firing happens at any time during the movement and not at the end of movement.

Except, that if I had finished my move facing engine to engine with your plane, the engine hits would have been tail hits. So that cannot be right either.

So, let me get this right.

You're making this up aren't you.

Now you almost have it. The phase is after movement but it takes into account movement.
If I'm moving in a line and you run straight at me you have to either have be running parralel to me or in the same track I'm running. Or, you are running at an angle and I can hit you as we go along. And I wouldn't really shoot you in the back. Just the side and have it blow through your back, before going out the other side. So actually I could hit the side facing away from me too.
This has been brought up in the past and the arguement is always the same and the end result is that the rules say that deflection means that you can hit anywhere.