Today few videos presenting some Polish musician - Cezik, who makes music videos with only him, but multiplied many times, so it seemes, that the song is played by much more people. One of the vidoes was made by singing "backwards" and then played backwards seems to be normal. This could not be described easli-just watch videos and enjoy.

Many (characteristic) Polish songs acapella

Singing backwards

Use somebody by Cezik