I have one of these, you simply pull a fuse and push the end in place and it will show you how much current is being drawn through that circuit even with the key off, it's good for finding that parasitic draw that drains batteries when not used for a while.You must be registered for see images attach
That's a pretty cool little gadget, I've always just used a multimeter with standard test leads but that looks way easier. Might have to look into one of those.