Desaturates the RBG based on the pixel data. The sensor collects the data and the filter ignores it. This would be the same pre or post processing as I understand it. So taking a photo in color and desaturating it post hoc would be the same on principal as setting it to black and white and taking it
Any photo you take can be presented in the other format. You can “take it” in BW and present it in color or vice versa. So while I took them in black and white, the camera was also taking them in color. The processor collects all data available to it.