To cut a counterbore on the far or inaccessible side of a workpiece by passing a specialized tool through an existing hole. The tool either has a spring-loaded cutter that deploys after passing through, or the operator manually attaches the cutting head on the back side before pulling it into the material. This technique avoids having to flip the part and set it up a second time.
Heard on the job
We need to back-counterbore those flange holes because there's no clearance to come at them from the bottom.