(Answered) When does a swarm attack begin?

[Eminog]

Tw pg 220 says "a unit performing a swarming attack may not be the target of weapon attack".  When is the unit considered to be performing a swarming attack, when the attack is declared or the turn after the roll for success is resolved?  Can the opponent declare weapons fire at the battle armor and then I declare them to be swarming and avoid the shot? 

Also, if an infantry unit is firing at a prone mech or executing a leg attack from the same hex, does it have a +1 or -2? the chart of Pg 117 of TW says "Target:  –2 from adjacent hex; +1 from all others" and the same hex would fall under all others so be +1, but this doesn't seem reasonable.   

[Hammer]

Tw pg 220 says "a unit performing a swarming attack may not be the target of weapon attack".  When is the unit considered to be performing a swarming attack, when the attack is declared or the turn after the roll for success is resolved?  Can the opponent declare weapons fire at the battle armor and then I declare them to be swarming and avoid the shot? 

A unit is considered to be performing a swarming attack the turn after it has rolled successfully to swarm.

Also, if an infantry unit is firing at a prone mech or executing a leg attack from the same hex, does it have a +1 or -2? the chart of Pg 117 of TW says "Target:  –2 from adjacent hex; +1 from all others" and the same hex would fall under all others so be +1, but this doesn't seem reasonable.   

-2. But beware of thrashing attacks.