I'm pretty sure Swype did explain it, and I know SwiftKey has this gesture as well (though I don't remember if it was ever explained to the user or just assumed they'd remember it from Swype).
Re:Google Memory Loss
by AndrewOMartin on Wednesday January 17, 2018 @09:10AM
(#16167963)
The default keyboard on my Android didn't have any such tutorial. I did look in the app and online.
edit: Don't actually see a tutorial in the app. Maybe I'm confused with another app such as Swype, but the same technique seems to apply to all.