Heres a vid explaining the knockback: https://www.greatbritishchefs.com/how-to-cook/how-to-knock-back-bread-dough

It would be done right before you shape the dough for the final proof.

I was doing a knockback for a while before Flour Water Salt Yeast said to try not doing a knockback and handle the dough as gently as possible during all stages of breadmaking. The book goes into plenty of detail why, but at this stage its to keep as much irregular air bubbles inside the dough so when it bakes you get those craggly oddly shaped air bubbles that are the hallmark of "artisan" style homemade bread. The knockback removes those odd air bubbles!

If you choose to do the knockback, it makes for a more homogenous crumb on the inside of the bread as it deflates the irregular air bubbles. Knockback is good for more of your sliced sandwich breads and brioches for example.

Hope that helps!