The strict definition of natural cheek line would be NOT to trim ANY of the hairs on the cheek. BUT if there are a couple of stray ones that are very high up and you want to get rid of them, then just go ahead and use a sharp pair of scissors and snip them off every few days. This way you keep the natural cheek line for the most part and you are never using a razor or trimmer on them and accidentally screwing up by trimming the rest of the cheek area too low.

The natural cheek line I equate to the beard's own signature. Every one is different and gives each beard a unique quality, even if the rest of the beard is trimmed and there is a defined neck line.