Mate 20: I can’t capture screenshots by knocking twice when playing games

Use your three fingers then slide over the screen same time...Then you can take screen shot...Setting-> Addition settings->Button and Gestures-> Then select three fingers screen shot

I noticed the knuckle knocking gesture works in some games but not all.

Check to make sure you’ve turned on the feature in Settings > Smart assistance > Motion control. If the feature is turned on, but it still doesn’t work, then you just got to
use the normal screenshot method - pressing the volume - and power button.