Why do knick points and waterfalls happen in valley long profiles?

Knick points are an irregularity in the rivers long profile.

Knick points occur in the river when river rejuvenation has taken place, meaning that either the land has risen or sea level has dropped, increasing the gradient of the river at that point. This gives the river greater potential energy and more erosive power. If the knick point is big enough, a waterfall would form.