Here are some illustrated illustrations of backpropagation from a course on learning artificial intelligence.
Backpropagation helps find the direction of the gradient. And combined with the chain rule, the gradient is calculated from back to front.
https://www.youtube.com/watch?v=wEoyxE0GP2M&list=PL3FW7Lu3i5JvHM8ljYj-zLfQRF3EO8sYv&index=6
Machine-translated from the Chinese original 反向传播图示说明 (published 2020-02-23). WeChat original ↗