It's called 'wombling'.
You take people's receipts from trolleys (has to have more than 8 items on there I believe), and enter code at the end on Asda Website. If you could've got the shop cheaper elsewhere, you get a voucher for the difference.
Someone apparently made £23 in vouchers this evening by doing it from her local Asda (trashed receipts on floors/in trolleys etc).
Has anyone else tried this?