An instance came up in an adjacent game that we couldn't figure out.
Player Drew teleported in 3 Terminator Squads. Player Scott had a buffed (BTS) Tactical Squad and 2 Devastator Squads on Overwatch. Drew Commander-Engaged the Tactical Squad with all the Terminators.
Scott fired all the shots into a single squad of Terminators, killing 3. The question was, is that unit forced to immediately withdraw (even though it's in base contact), or does it fight as a broken unit?
Unless I've got it all wrong. Overwatch says "A formation that is on overwatch may choose  to shoot immediately after an enemy formation completes a move or unloads troops, and before the target either shoots or assaults."
But the Assault rules have Charge Move as an internal component of Assaults (Subsection II). So if you have to shoot before the assault, when does this happen? Before the Charge move? How does Counter-Charging interact? Can I have an assaulted unit on Overwatch fire and then CounterCharge? Can I have a unit CounterCharge before a friendly Overwatch fires? Heck, can I have a friendly Overwatch near my assaulting units shoot at a CounterCharger?*
* This was mentioned in another thread that countercharging/consolidation are 'movements' and are treated as such for certain effects.
Or have I got this completely bass-ackwards, and Overwatch fires before the movement of the Engaging unit. "A formation that is on overwatch may choose to shoot immediately after an enemy formation completes a move or unloads troops, and before the target either shoots or assaults." So a unit charging from out of LOS can't be overwatched? Because the opponent cant see him before he begins?
I'm really confused. ÂÂ

 And getting more confused the more I think about it.
Morgan Vening