It's pretty easy. The defuser will defuse any explosive device. Just select the defuser, and there will be an icon in the middle of the screen where the reticle usually is (like the assault defib.) Place the icon over a motion mine, emp mine, etc. and left click. It will disappear like magic. Very handy for when you're riding in a vehicle being a backup engy.

It performs pretty much the same function as the motion mine bait does. But there are some differences. The motion mine bait can be used at a distance and thrown, the defuser is more like a melee range weapon. You have a very limited supply of MMBs, but the defuser can defuse any number of items since it isn't ammo based. So you're essentially trading exposure for repeatabilty, but it's pretty easy to pop out of a vehicle, load up the defuser, and then zap all the mines you've managed to attract. The motion mine bait will be better if you spot a field before you run into them, and the defuser is better if you're already in the middle of the field due to the proximity of the mines especially if you've drug them enough to clump up (both from an ease of disarm perspective and the final MMB blast might endanger the vehicle if it is too close.)