I used a First York bus today and did tap on / tap off. The driver told me to tap the card on the card reader at the cab (the same as normal pass holders or card payers would use) and I tapped off the same, only realising afterwards maybe you are supposed to tap and at the drivers machine and tap off at the contactless pad as pictured on this thread?

I'm assuming although different operators, the way you do it should be the same? Anyway, it worked with me using the drivers machine twice, and I'll see what I'm charged !