As far as I understand it, the trigger knob on the oscilloscope just adjusts at what voltage the scope will trigger the horizontal sweep (whether is be on the falling or rising edge using the +/- slope switch). Therefore I would assume it would just shift the waveform left or right on the screen, but the trigger level knob actually allows a messy looking signal to be adjusted to a single clean waveform to be displayed on the screen. How does it do that?