Yakface has taken the time to clarify what the attack is with a pretty good description. Now if you know more Yakface, since apparently you do, email me..... lol.
Remember that these are still only rumors, please take with salt.
Okay, a much more full description of the rule is like this:
This rule applies ONLY to the units the special Necron character is engaged with (it doesn't affect every model in the enemy army of the same type).
It happens after the Necron character has killed someone in combat and after all blows have been struck on both sides.
Roll a D6 for every type of model (friend or enemy) that the special character killed that turn. On a 4+ all other models in combat of that type take a wound (armor/inv saves can be taken as normal).
Example: If the Necron character kills an Ork Slugga boy in close combat and got the 4+ roll, all other Slugga boyz in the same combat would take a wound, but NOT the nob (as he's a different type of model). Any Shoota boyz, for example, in the same combat would be unaffected as well. If he killed a marine and got the roll, all other marines would take a wound, but not the vet sgt, etc.
It would seem that if this Necron was fighting against another Necron player, then he has the potential to hurt his own forces if the same type of units were facing off; if he was attached to a unit of warriors that was fighting against an enemy unit of warriors, for example.