You don't play the ANS synthesizer with a keyboard. Instead you etch images onto glass sheets covered in black putty and feed them into a machine that shines light through the etchings, trigging a wide range of tones. Etchings made low on the sheets make low tones. High etchings make high tones. The sound is generated in real-time and the tempo depends on how fast you insert the sheets.
Pretty fascinating, considering it was created in 1938. It was also used in the score for Tarkovsky's film Solyaris, and the band Coil travelled to Russia specifically to use this old instrument for an album. (The sheets in the image above are from their sessions with it.)
More Information at Wikipedia: ANS Synthesizer and Audio Sample