While some folding treadmills may have more safety features built into them, there are two that are absolutely vital. The first one is a lock to keep the deck from falling once it is folded up. If the deck were to fall, not only would it most likely cause damage to the machine, but it can injure you as well.
The second is a safety key which would need to be inserted into the treadmill for it to power on and operate. This keeps anyone, especially children, from accidentally turning it on and hurting themselves when the belt starts turning.