Documentation for SPI protocol between Pico and Pico Wireless

I’m playing around with Pico Lipo + Pico Wireless and am trying to program it in Rust. Which means, I need to implement the protocol between Pico and Pico Wireless. I’m using the source code here as a guide: pimoroni-pico/drivers/esp32spi at main · pimoroni/pimoroni-pico · GitHub. It’s mostly pretty clear, but I’m running into some problems, and would like to read some doc on how this SPI protocol is supposed to work. For example, how exactly does the ack/ready GPIO pin is supposed to work and why some functions switch chip-select back and force.

I’ve tried looking at ESP32 reference manual, but couldn’t find the description of this protocol.