The factor which determines if a Rebounder is a bad, ordinary or high quality is the
Let me explain: In the picture above you see four different colored lines which are all moving in an arch above and below a horizontal line. These lines demonstrate the movement of different Rebounder mats while bouncing. The part above the horizontal line shows the up-movement of bouncing and the part below the horizontal line shows how the Rebounder responds to the down-movement of bouncing. The horizontal line itself represents the Rebounder mat without bouncing.
If you look carefully, you see that all lines except the red line have the exact upper and lower amptitude (or arch). What you see when you look at the red line is that the up-movement shows an arch while the down-movement shows a "bump". That means if you bounce on this kind of Rebounder, you will move up during the up movement, but during the down movement the mat does not give. Instead the mat stops the movement and you receive a jolt in your back. This is how a "bad" Rebounder works. The reason why the mat does not "give" is due to cheap springs and low quality mat material.
Only use a Rebounder where the upper and lower bouncing amptitudes are identical in size and shape. This assures that the mat cushions you as you approach the deepest point of your bounce, protecting your back from any jolting or jarring.
On the graph above, which line represents which Rebounder?
The red line represents a cheap Rebounder from a sporting goods store. Unfortunately, Rebounders of such low quality are the most commonly used Rebounders here in the US, but they actually do more harm than good. The blue line represents a bungee band Rebounder like the Jumpsport 370 which is harder in bounce then the bellicon but softer than a spring based Rebounder. You see that it is not as "bouncy" as the yellow or green line, but it is a very good Rebounder that gives you a good workout without the threat of injury. The yellow line represents a bellicon with strong bungee bands, and the super soft green line represents a bellicon bungee band Rebounder with medium bungee bands. All these high quality Rebounders (blue, yellow and green lines) absorb 85 percent of the impact and leave you with only 15 percent. This is the feature that allows almost everybody to exercise on a Rebounder safely—at any intensity.