The spring is coiled inside the pipe, attached to a spiral rod which is inside the spring. as the window is lowered the spiral twists the spring and creates tension on it, which gives it the force to assist in raising the window back up. There are some cheap lightweight balances , which are useless and should not be bought, even if the better companies sell them. They will fail. Mightons have a reckoner. Which is okay if the weight of the window is close to the minimum weight given for the balance. but if its at least half of the quoted range go up to the next range. they will have enough power to lift the window.