what the researchers achieved was to sense movement finely enough to distinguish American Sign Language down the the digit level at better than 90 per cent; and better than 82 per cent for “single individual number text input”.
[…]
The researchers say the “micro motions” involved in finger gestures cause “a unique pattern in the time series of CSI values” (dubbed “CSI waveforms” in the paper), and those waveforms are unique to the gesture.
[…]
Right now, WiFinger imposes constraints on the user – rather like the gesture recognition on the Heart of Gold (The Hitchhiker’s Guide to the Galaxy), it seems you have to “sit infuriatingly still” for the system to work.

Source: Text input from thin air: boffins give Wi-Fi the finger with AI