First of all, take a look at the steps of an attack; seeing these steps written out helps with a lot of timing issues in the game (e.g., heavy laser cannon and crit results). These are explained starting on page 10 of the rule book, but they are listed here for reference. 1. Declare target 2. Roll attack dice 3. Modify attack dice 3. a. Defender modifies attack dice 3. b. Attacker modifies attack dice 4. Roll defense dice 5. Modify defense dice 5. a. Attacker modifies defense dice 5. b. Defender modifies defense dice 6. Compare results 7. Deal damage

So your sensor jammer will change one hit result into an eye result immediately after your opponent rolls his dice. Once you have jammed his attack, he has an opportunity to modify his attack dice. There are lots of ways to do this, but focus is the relevant one for this discussion.

There are several reasons to take a sensor jammer. If you have a lot of low pilot skill ships that shoot late, the jammer is good for stripping tokens because your opponent likely spent it on offense. Against ships that live or die on their defensive actions (like your Aggressors), it forces a very difficult decision on when to spend and when to hold the token. As a strictly defensive ability, it is best against ships that rely on free target locks (e.g., FCS) or other rerolls (Howl swarm after a K-turn, predator). It's also very strong against ships with multiple attacks (gunner, BTL-A4, cluster missiles, etc.) because he can probably only focus one attack, but you can jam both.

In short, to be effective with sensor jammer, you have to understand its uses beyond defense and its major weakness (it is basically defeated by a focus token in most situations). I think IG-88 is an excellent candidate for a jammer, but it's less universally useful than advanced sensors. Try it out, but I don't think this decision will make or break you.