When do the effect of the MS-1 Ion Cannons take effect if you select to exhaust your opponents ECM upgrade card?

Is it immidiately before he has a chance to use it?

Or may he select to use it first, as its part of the spend defence token step?

Obviously the ECM card will be exausted before any secondary or follow up attack is made

When do the effect of the MS-1 Ion Cannons take effect if you select to exhaust your opponents ECM upgrade card?

Is it immidiately before he has a chance to use it?

Or may he select to use it first, as its part of the spend defence token step?

Crit effects are AFTER spend defence token step.

So he can use it.

Indeed it doesnt trigger till that point, so you could just do a standard crit effect instead if you wish at this point, if he has just panic discarded the brace....

He may elect to use it first, as it occurs when the target ship spends its Defense Tokens. Critical Effects are not resolved until after this step.

It's a good option though if you want to hit it with a ship that won't do much damage, such as a CR90B or Raider-2. ECMs tends to be a really important strategy card for surviving big hits.