Fulda Gap 85 - Helo Question

[Kool Kat]

Gents:  

In the Modern Series, when you use a ground unit to "assault" a helo, the message "Assault Overrun" is displayed and the combat result is your ground unit displaces the helo to an adjacent hex and the ground unit then occupies the hex vacated by the helo with no losses to either side.

What exactly (in battle field terms) is happening here?  

How can ground units "assault" a helicopter that is in flight?

Any Modern Series vets who can add insight?  

Thank you all in advance!