One teacher, one student, no class. You speak, you are pushed into the tasks you are worst at, and the last ten minutes are the recording played back with the habits costing you marks named.
Booked singly or in blocks of five, and a block is cheaper and never expires. Exam students book them in the fortnight before a test; other people book one a fortnight for years.
The habits that hold a speaking score down are rarely vocabulary. They are filling silence with the same three words, answering a forty word task in five, and repairing a sentence out loud three times instead of finishing it. None of them are visible to the person doing them, which is why the recording is the point.