When the arm is slightly out of tangent, a laser beam and sensor detects this condition and a servo motor slowly rotates base that the arm is on to reestablish proper geometry.
So in a nutshell it is just like digital - only a little bit out all of the time ( apart from the start ).
Reminds me of the Goldmund linear tracker waltzing across the record -
2 steps forward, 1 step back, hippity hoppity boo, I see a new cartridge for you.
At least with the Goldmund you had a flashing led to show you how often the arm was out ( most ).
I'll stick to my air bearing linear tracking Eminent Technology ET2 if I want zero tracking angle error.