What is Una Corda?
It's a custom made piano by german manufacturer David Klavins.
https://www.youtube.com/channel/UCxLzutIv92Pe3ZGqoS42VmA
The main feature of that instrument is that each one string (88) is struck only by one hammer, creating incredible clarity
and distinct sound that differs of usual piano. The large grand concert piano can have up to 230 strings struck by 96 hammers each hammer hits 3 or 2 strings each time the key is pressed. Typical medium piano does have 230 strings and total pressure of strings combined can reach up to 18 tons.
Una Corda is also
"a device in a piano that shifts the mechanism slightly to one side when the soft pedal is depressed, so that the hammers do not strike all of the strings when sounding each note and the tone is therefore quieter."
http://www.dictionary.com/browse/una-corda
The Una corda piano was ordered by known pianist Nils Frahm.
Listen to his music here
The custom piano was presented publicly in 2014.
This is beautiful and haunting performance by Lisa Morgenstern on Una Corda in Hungary 2017