Prestretching is done to reduce initial tension loss and coil memory. Some electronic machines have this feature built in. It involves over-tensioning the string by a preset percentage and then dropping back to the reference tension. Off the machine it is usually done by tieing one end to a stationary object like a pole or door knob. The string can also be wrapped around a padded pole or door knob or pully so the string is only 1/2 as long. Then the string is stretched out to its full length(or the two halves) and held with parallel jaw pliers, a starting clamp or wrapped around a block of wood(anything that will hold it without slipping). Then you pull on it hard until you feel it relax. This is the point where you feel the string firm up. It is important to be consistent with the length of time that it is pulled in this way. This usually takes about 30 secs. or so. The string will be stiffer, but less powerful after you do this. Make sure the string doesn't slip out of whatever you're holding it with or come untied. It can literally dump you on your keister or snap back in your face.
I haven't fallen on my butt (yet) but I tie knots in the last 1/2 inch of the string at each end, then grab them with a pliers (or vicegrips) and generally pull on the ends with a piece of cloth (old t-shirt) over the knots for extra grip. Since starting the knotting of the ends it hasn't slipped out.
#when pre-stretching string use your starting clamp. tie one end of the string to a secure object, then thread the string through the eyelets on both sides of your starting clamp, then open the clamp and pass the string through the grippers. Rest the base of the starting clamp in the palm of your hand and with the other hand hold the clamp upright and pull the clamp towards you. DON'T SQUEEZE THE CLAMP, OR YOU RELEASE THE STRING.
If you are doubling the string and need to hold both ends of the string in the clamp, put one end of the string through each of the eyelets and then put both pieces of string in the gripper.
This method saves having to tie knots, use cloths and anything else.
I've tried differnt methods(starting clamp, parallel jaw pliers, by hand, blcok of wood) and found the most secure method for me is to use a block of wood(a 6" section of 1"X2" works well) with two holes drilled in it about 2" apart. I thread the two ends of the string through the holes and then I turn the piece of wood until the string is wrapped around it 2 or 3 times. I also stand sideways to the string. I figure if the worst happens and the string does come loose I am less likely to fall or be struck in the eye with it.