Preparing the wire
The wire reel mounting normally includes a spring tensioner. This tensioner should be initially tightened to the point where the reel of wire doesn't unravel under it's own spring tension.
The first 3 inches of wire should be as straight as possible to reduce the chance of damage to the liner or snagging as the wire is fed through. Sharp wire cutters can be used for trimming.
Letting go of the end of the wire would cause it to unravel and tangle. (In the photo the hand normally used to hold the wire when cutting is being used to operate the camera.)