I know exactly what you are talking about and I have no idea how to access that checkpoint without assistance from an airport employee.

Twice I have been directed by TSA to that metal detector only checkpoint and once an airline employee was helping me make my flight (was really late) and after having my docs checked she yelled he is late late late and needs to be screened ASAP.

She lead me into that "special" checkpoint and I was screened there is no time flat and barely made my flight.

Dan