You could find some explanations in the section 10.3 Power On, Reset, and Boot in the WF200 datasheet :
Moreover there is also some details in https://www.silabs.com/documents/login/user-guides/ug382-wf200-hardware-design-ug.pdf in section 4,.
When device is not supplied yet, pin RESETn shall be set to LOW and No HIGH voltage is expected to be applied to input pin LP_CLK.
Once power supplies have settled, a delay of 100µs is recommended before releasing pin RESETn to HIGH.
When powering down the device, it is recommended to set RESETn pin back to LOW before shutting down its power supplies.
The power down step sets the WF(M)200 in standby mode according to these two ways below: