To start: toggle both switches to the outer position.
A little audio phenomenon just to mix things up. When 2 sine waves of equal amplitude and close frequency superimpose the resulting wave experiences 'beats' where the amplitude changes as a sine function with frequency equal to the difference between the original frequencies.
Here the inputs are a 50Hz and 60Hz wave with almost equal amplitudes, and the resulting 55Hz has a 10Hz beat envelope.
If you've ever tuned an instrument this is what you're hearing when the notes are extremely close, e.g. a 439Hz note and a 440Hz note played together will fade in and out of full volume twice a second (since there are 2 amplitude peaks in each beat period).
