Instruction clarification

[Teiris]

Hey all, I need some help deciphering these instructions. It's to make a knee indent on a doll leg.

"Pinch leg to determine where you want knee. Put yarn through at that point going straight through to other side of leg. Go over one strand of yarn (so yarn won’t pull through when tightened) and go back through leg to starting point. Pull yarn ends until you have the amount of shaping that pleases you. Tie in knot and put ends through doll’s leg."

What does "go over one strand of yarn" mean?