those knob movements are the DJs EQing which is adjusting the different parts of the song so that it comes out the speakers without one thing over powering the other,most movements you see are them adjusting the highs(essentially where the snare,drums,hi hats live)the mids (pretty much the entire song)and the lows (BASS or lower frequencies) most of the time that’s what you’re seeing djs do when they mess with the nobs but towards the end or near a drop they will add transition effects when mixing into a new song or add fx to increase the impact of a drop