I think you are supposed to have your foot angled slightly away from the bike to make sticking your knee out a little easier, so heel touching your heel plate and toes nearer the end of the footpeg
It is actually really easy when you try. My mate was a bloke that always went round a corner As upright as possible. After a few minutes and a adjustment Of his riding position he was draggin knee. Needless to say he was doing it at every opportunity on the Way home. He still is struggling to get his left knee down He hasn't got the nerve to push that little bit needed to Touch down.