Tesco club card app vouchers scan at till trick

When you scan the club card app bar codes on phone at till (self scan service I use) the code works but a message pops up on screen asking for paper voucher. At that point wave something in the voucher slot that's at knee height. I used my bank card but anything like shopping list will work. The staff use a plastic pass they carry around neck or bit of card and move in slot. I was told anything works and a quick left and right movement of my bank card got me a voucher accepted voice. Flaw in system I guess.
Result.