Thank you for question. I am glad that you like pipe organ music.
That handles which I pushed in playing time is switches of organ stops.
Organ stop is color of sound. Like painter have variuos color of paint (also he can mix that colors and make new color) organist have sound colors - organ stops. Inside organ is many pipes. For example this organ in recording has 566 pipes. For manual this organ have 8 stops and 2 stops for pedal. In manual keyboard is 54 keys. So for one stop is 54 individual pipes for each key. In the other stop there will be another pipes but that pipes is not same as in first stop. There can be metal pipes, wood pipes, can be variuos diameters for same lenght pipes and etc.
Some of organ stops demonstration examples you can hear there: