Foot (shoe) slips out of the pedal?

Tighten the ankle strap (opposed to toe strap) is the key. Make sure to find the tightest possible angle when you secure the velcro, too. If you still have problem, applying something to secure the heel position will help.

Most popular method among our users is using a part of leg strap.

This is leg strap. One of the centre part can separate like this.

Here below is the heel part of the pedal. use the above part to secure the heel.

COGY's leg strap is made with wide and thick neoprene, which has right amount of stretchiness. It also has velcro with high adhesiveness. These make this little pice suitable for heel strap. Alternatively, I would recommend using the therapy stretch band and make a knot to tighten.