With two Saf-T-Flex units back-to-back, you have three ways of using the leash depending on how much strength is required.

HEAVY
With both Saf-T-Flex units closed, you have maximum strength and resistance.

MEDIUM
Open one Saf-T-Flex; the other unit continues to absorb tugs and jolts.

LIGHT
Open both Saf-T-Flex units for lighter resistance when walking smaller dogs.
Train your dog not to pull!
Start the dog off on the HEAVY setting (both Saf-T-Flex units closed) for a few weeks. The dog will no longer be pulled off balance by the owner, so it is more comfortable=less pulling. Remember, hold the handle firmly in one position. Do not jerk the dog's head or the dog will continue to pull as before.
Once the dog is pulling less, release one set of Saf-T-Flex units for the MEDIUM setting.
When the dog gets used to that setting after a period of time then unclick the remaining Saf-T-Flex's, achieving the the LIGHT setting. After the pet gets used to that, the dog no longer pulls. You've just saved a lot of money on training costs.
NOTE: Only change the leash settings after the dog has shown significant improvement.