Let’s assume for arguments sake Tommy Fleetwood is bent forward 40 degrees at setup.

By the time he reaches P4 (top of the swing..) his spine will have extended (bent backwards) close to zero degrees. That’s 40 degrees bending backwards…

From P4-P6 the spine then flexes again (bends forward)..

..and from P6, through and post impact, the spine is continuously extending (bending backwards).

Here’s some further explanations and good examples.