1) Insert Disc #2 into your CD ROM
2) In the "DOCS" folder, you'll find the Manual.PDF, and the QRC.PDF "Quick Reference Card."

3) Here's what the manual says about ECM:
Electronic Countermeasures (ECM)
Electronic countermeasures protect Alliance fighters from
turrets found on enemy capital ships by scrambling sensors to
prevent weapons lock. To activate ECM, press L. ECM has a
limited duration before it must recharge.

Watch your 6!

Using ECM against the Ion Cannon is an interesting idea. Explore this in Single-player (if I don't beat you to it) and let us know back up here. I'm really curious to see if it works. On the missiles note, spam out counter-measures to decoy missiles. I'm not sure what ship/difficulty your using, but I've found that I'm usually dead by the third missile hit, hurting badly after the second, and not in very good shape after the first.

I don't play much single-player though, I beat the game through the co-op team play long before I ever played through it on my own.

I am playing are hard right now (and am getting quite frustrated by the Boradin mission -- dang Rippers that keep getting nailed -- but that's something else) and am trying to figure out what effect enemy missiles have too. I can normally get away with one countermeasure launch or I can just keep flying and nothing happens (unless it is a Havok).

I am unable to get on Starlancer at the moment, so I can't test the ECM on the ion cannon. It seemed to work in multiplayer though. It was Shroud vs. Phoenix on Dark Reign deathmatch. The person that I was playing with had been locked on by the cannon but then hit his ECM and somehow the ion cannon lost the lock long enough for him to dodge up out of its line of sight.

"Writing is a socially acceptable form of schizophrenia." - Unknown

I've tried ECM vs Ion Cannon...
No-Go!
Sucker still takes you out!
Except for distance, another ship/object between you and the Ion Cannon, the only thing I've found that works every time against the Ion Cannon is the Shroud's Invisibility mode.
You can decloak long enough to launch a missile or two, or take out an enemy ship/turret, before Moose tells you that it's locked on to you again.
Once Moose yells at you, you've got 5-10 seconds before it blows you away!

Watch your 6!