Why we chose a double press of Shift to change the keyboard layout of the last word.
A hotkey is usually a combination of two different keys, which requires two fingers or even two hands, as well as some thought — because you have to remember where those keys are. Any key combination will be inconvenient for someone in one way or another.
By contrast, you can press Shift twice with one finger. Since there is only one key and everyone knows where it is, very little mental effort is needed — it is easy to get used to this hotkey. Most importantly, the action takes very little time, less than a second.
Why Shift?
We are sometimes asked why we chose Shift for switching the keyboard layout, converting selected text or undoing a switch.
First: Shift is a key that is in the same place on any keyboard, in any operating system. We made a universal solution for people who work on different computers and operating systems, saving them from confusion and from having to recall reflexively what is where and when. One key is easy to remember, especially when everything related to layout switching is concentrated in it.
The second important point is that there are two Shift keys, left and right. This doubles the key’s availability for people who have physical difficulties using their right or left hand.
Third — pressing Shift does not put any characters on the screen. It is a modifier key, free to be used for switching because it is used in combination with other keys for any other action.
Fourth — an important point — Shift is the second, sometimes third, largest key on the keyboard. It is very easy to hit :)