Picture a conventional Bench Press. The chosen weight (let's say 135 pounds) is exactly the same at the first push as it is at lockout. Now imagine 135 pounds at the initial push amplified to 175 pounds at the top. This is the idea behind variable resistance training.
Bands impart greater resistance as they stretch and build tension.
The first and probably most pronounced benefit is the increase in power output. The increased intensity of each exercise near the lockout phase requires a buildup of speed and force through the rep. Unlike the standard resistance of gravity only, you won't feel any let-off near the lockout phase where momentum and muscle force are typically highest.
The second benefit is increased muscle stabilization. Standard reps are smooth and stable. In contrast, variable resistance feels like exercising during an earthquake. The inconsistent elasticity of bands and the tendency of chains to swing cause the body to recruit many muscles to stabilize the weight. For example, during the Bench Press the abs and obliques are called upon to keep the torso rigid so that the chest can maximize its power output.
A third benefit is increased eccentric load. The eccentric phase of a rep is the lowering portion, when the muscle lengthens while controlling a resistance and lowering the bar during the Bench Press. Variable resistance creates the highest load at the top of the movement just before the eccentric phase begins. This increased tension requires the body to fight harder to prevent the bar from crashing down like a stone.
These benefits add up to tremendous results. The increases in power, stabilization and eccentric activation lead to greater muscle damage, and as a result, bigger, stronger and more powerful muscles when they rebuild to make you the best football player possible !