Hi, SCB's regenerate an active shield by a certain amount. The original default key is S to activate them. If the ship's shield has collapsed it has no effect and won't revive it, you have to wait for the shield to start regenerating before it becomes effective. By how much depends on the module's rating for shield reinforcement per second and it's duration, which varies greatly depending on it's module size.
They can be very useful to get you out of a tight spot but come with the drawback of generating large amounts of heat, which will often take you over 100%. Exactly by how much and how long it takes to dissipate depends very much on the ship type and power plant rating. The other drawbacks as with most modules is the use of a slot, it's mass and it's power draw.
You can use a heat sink at the same time to contain the heat build up which is even more important if you use more than 1 at a time. At the moment the only way to avoid using more than 1 is to turn the other(s) off and switch them on when needed. I've found if you use a lower size than the ship's shields, you can sometimes "tough it out" without a heat sink as the overheating only lasts a few seconds without affecting things greatly. The downside of course is you probably won't get the shield back to full strength straight away even with 4 pips to sys but it's better than nothing and might just save your hide!
Bear in mind also it does NOT add to your shield strength the way shield boosters do, they just try and revive it as far as its rating allows.