I'm trying to build a clock as in the tutorial, but am confused with part of the code, and why my example doesnt work quite as the tutorial does.

I understand all of the code up until the highlighted part, at which point I lose track of what's being done. If anyone could explain this last part to me, and what function I should try using to remove the 'old' second-hands from the clock I would much appreciate it! L.

        void setup() {
      size(800, 600);
      background(254);
      smooth();
      frameRate(20);
    }

    void draw(){
      smooth();
      noStroke();

float s = map(second(), 0, 60, 0, TWO_PI) - HALF_PI;
stroke(100);
    strokeWeight(1);
    line(400, 300, cos(s) * 200 + 400, sin(s) * 200 + 300);
}

Thanks!