The manual discusses this:
https://carbide3d.com/hub/docs/65mm-vfd-spindle/
pg. 14
Press the Spindle Enable button on front of VFD controller to enable the spindle. The spindle is enabled when the button is depressed and the button LED is on. The spindle is disabled when the LED is off.
I believe best practice is to press that to release it so as to remove power from the spindle when doing a tool change, then press it to engage it after.
I will yield to the folks who actually have a spindle — I use a BitRunner w/
because I invested in it before we offered a spindle.