Unknown Features On The Xiaomi Rom By Pressing Home B

[Hobbes.]

Hello !
I use a Xiaomi Mi5 with ROM 7.1.19, and there is a feature I did not know:

When the button is pressed, there is a white circle and a white rectangle that appears.

Do you know what it is for? What can we do with it?

Thanks !

 

[madamaszek]

You can specify this button under additional settings. By default it's a Google Now.