The second person (2) stands the crossbow with the stock (tiller) on the ground so that the crossbow aims straight up.
Then (2) presses on both ends of the bow with equal force only until one string loop can be removed from one bow end. Please be careful in this moment. If you exert too much pressure, you can damage the crossbow bow or even injure yourself.
(1) removes one end of the string after another and puts the new string on - again the first end and then the other. (1) should work quickly so that the forces of (2) are sufficient to hold the bow under pressure. The new string should be twisted exactly the same as the old one before it was replaced.
After putting on the new string (2) can release the pressure slowly and (1) controls that both string loops are placed correctly in the lathe grooves.
Alternative method how to replace the crossbow string
For an alternative proven practice you need a strong strap or belt. You can replace the string of your crossbow alone without the help of another person here.
Pull the crossbow bow (lathe, prod) and secure it with a strong strap or belt according to the sketch on the right. Then release the old crossbow string from the revolving nut and replace it with a new one. If the old crossbow string is not installed for some reason and you cannot surpass the draw weight of the prod with your own power, reach for a winch to draw the strap. Proceed with caution not to get injured!