Our Hand Drive wheelchair attachment uses a dual ratchet mechanism. One ratchet is naturally engaged by a spring and moves the wheelchair forward as you pump the handles. To move backwards, the user squeezes the brake on the handlebar which disengages one ratchet and engages the other, and the wheelchair moves backwards.
Because it is open source, it can be adapted for individual user's needs. For instance, if the user is a child and they have smaller hands, they can adjust the handles and handlebar length to fit. If the user has unusual wheels on their wheelchair, then they can adapt the attachment piece so it can fit snugly on their wheel as well.