Securing cables going in/out of plastic boxes

I understand that for plastic receptacle boxes, code requires the cables to be secured (e.g. stabled against stud) within 8" of exiting the box.

However... is this 8" of "wire length", or straight-line distance?
I assume what's important is the intent - is this to serve as strain relief for the wires inside the box and connection to the fixture?

For instance, magine wire coming through a hole in a stud say 4" above a receptacle, bending UP and being stapled there (say 3" above hole) then there is another say 12-14" or wire looped and going down into the box. This would make it easier (1) to push the wires up into the wall after wiring in the outlet, and (2) plenty of extra if needed.
result 7" between wire and staple, 14" actual wire length.
However it kind of works against the idea of strain relief...

 

The romex cables are stapled just before they enter the box. The cables are stripped so that appox 1/2" of the sheath is inside the box. The actual wires should extend at least 6" out of the box. After connecting the receptacle.... the wires are rolled into the box. The cables are not slid back out of the box.

 

The sheath needs to extend at least 1/4 inch inside the box. The extra conductor is not pushed out of the box but is accordion folded into the box. You need at least 6 inches of free conductor in the box past the sheath.

A service loop is not needed. The cable is measured along the sheath distance.