Mogg Fanatic and Flanking

[RoddyVR]

If my sidewinder sliver (flanking 1) attacks, and the opponent has a Mogg Fanatic in play.
Can he block with the fanatic (which the sliver's flanking would make into a 0/0) and still sac the fanatic to shoot it at my head? or will the fanatic die as soon as its declared a blocker (for being a 0/0) and not have a chance to activate its ability?

[Ewokslayer]

Your opponent will have an opportunity to sac fanatic for its ability after he declares fanatic as a blocker but before sidewinders flanking ability resolves.
If your opponent lets the ability to resolve he won't be able to do anything.

[ninjabear]

As Ewo said.

Flanking is a bit confusing because it seems something "automatic", but in fact it's a triggered ability that uses the stack (few static critter abilities use the stack). Look at the reminder text:
Flanking (Whenever a creature without flanking blocks this creature, the blocking creature gets -1/-1 until end of turn.)
As a rule of thumb, abilities that use the words "Whenever", "When", or "At" are triggered abilites.
So when Mogg is asigned as a blocker, the Flanking trigger goes to the stack. Before the trigger resolves, he can sacrifice the Fanatic to deal 1 damage to any valid target, and the flanking critter would be already blocked.

He cannot, however, assign damage as a blocker, and then, with damage on the stack, sacrifice the fanatic, because the flanking effect has been triggered and resolved before the fanatic has a chance to put his combat damage on the stack. So for example if your sidewinder has a toughness of 2 for any reason, there's no way for a lone 1/1 fanatic to kill it (can't block, leave damage on the stack AND sacrifice).