A common error that is seen on new riders is that their feet are heel high. Note the photo of the runner. He is on the start line ready to explode forward as fast as he can. The heels are lifted, the pressure is on the toes and he is coiled like a spring. This is a perfect position for the fastest move forward off the start line.
This photo is an extreme example, but when you see a guy on a bike riding with his heels higher than the balls of his feet then he is in that similar position. If anything goes wrong he will propel forward, and a classic “over the bars” (OTB) is often the result.
When riding on non challenging terrain the feet should be flat, ball and heel level. When riding technical or challenging terrain including the steep descents, drop the heels below the balls of the foot. This action will bring the waist towards the rear of the bike, and if everything goes pear shaped. Its far less likely for an OTB to occur.