The TerraCycle Easy Knees Crank Shortener allows you to shorten your pedal stroke. The default crank arm for most bikes and trikes is 170mm or 175mm, which is often just a bit too long. The Crank Shortener can be very useful in relieving strain if you simply don't have very long legs, if your legs are different lengths, or if your knees have a limited range of motion.
These are also a great way to try out different crank arm lengths before you purchase a new crank arm set. You can make sure you invest in the perfect set for you!
Pro tip: if you have one leg with less range of motion than the other, check out the Easy Knees Pedal Swing. It lets you maintain the pedal stroke for your "good" leg, while adjusting the other leg to a stroke that's manageable for you!