Inertia is the resistance of a body to change in motion, and is a function of mass.
We have a 90-kg collegiate volleyball player who’s about to perform a vertical jump. Theoretically, they have to produce more than 90 kg or ~900 N (90 kg x acceleration due to gravity = 9.81 m/s2) of force into the ground to get airborne.
If we put a vest on them and place 10% of their body mass (9 kilograms) on the vest, then they would have to produce more than 99 kg or 990 N of force into the ground to get airborne.
The resistance to change in motion—as it is taking more force to produce motion.
With vest loading, that is easy to understand. With lower limb loading, however, we have an additional complexity in that the thigh and shank rotate around the hip and knee, so WR provides a rotational overload.
Many people will read this and not think much of it; however, this is what makes WR pretty unique.
It provides a direct rotational overload of the muscles.
Even though a 100- meter sprint is a linear activity, getting to the finish line is the product of rotation at the legs and arms, so rotationally overloading the limbs used for sprinting makes a lot of sense.
It goes back to maximizing specificity to optimize transference.
Check out the article & overload through rotational inertia: