Hit and run

I recently had it suggested to me that deffkoptas could possibly use hit and run after assaulting a tank. The benefits to being able to do this would be amazing the biggest reason my deffkoptas die is that I tend to use them as a first turn missile to take out the enemy tank/transport of my choice. However after the assault they are stuck there, surrounded by enemies waiting to be killed. Were they able to hit and run after that even if they still died it would be more work for the enemy force. They would have to chase them and get out of position the deffkoptas would have at least served a second purpose.

However as I look at the rules I would say that this is a gray area in the rules. Hit and run says a unit locked in combat can hit and run at the end of the assault phase. The rule book however fails to ever clearly define "locked in combat" it gives situations where things are or are not locked in combat, like in assaulting tanks it says that units in base to base with a tank can be shot because they are not locked in combat and in the movement phase either unit can move away because they are not locked in combat but it does not sat that they are not locked in combat during the assault phase which is when the rule takes effect. I would say once you attack a unit if nothing else for that one phase you are locked in combat. If not you would be able to assault something else but since you locked your unit into an action which was combat they can make no other action.

I can also see the other side of this where if in the movement phase and the shooting phase you are not locked in combat why would you think that you would be in the assault phase.

Another thought I have had is when is the end of the assault phase? Is it after pile in? If it is after pile in I have seen a lot of people use it incorrectly.
Say I have two units in a fight one breaks and the other is my deffkoptas who are using hit and run does the enemy get to sweep the unit that broke or does the end of the assault phase happen after we stopped them from being able to do that?
I would say the end of the assault phase is after all assaults are complete all saves had been made and all morale checks/sweeping advances have been made. You then have the option of using hit and run with all units that have that universal special rule. I have never seen it played that way but that is how I would think that it is meant to be played.