Javascript code written below is displaying numbers on an analog watch, but i am not able to understand the logic of for loop mentioned in the code. Can anybody please explain this logic?

function drawNumbers(ctx, radius) {
    var ang;
    var num;
    ctx.font = radius*0.15 + "px arial"; //set font at 15% of radius
    ctx.textBaseline = "middle"; //set text alignment to middle
    ctx.textAlign = "center"; //set text alignment to center
    for(num=1; num < 13; num++){ //calculate the print position for each number
        ang = num *Math.PI /6;
        ctx.rotate(ang);
        ctx.translate(0, -radius*0.85);
        ctx.rotate(-ang);
        ctx.fillText(num.toString(), 0, 0);
        ctx.rotate(ang);
        ctx.translate(0, radius*0.85);
        ctx.rotate(-ang);
    }
}

i tried to debug the code with the help of browser but i was not able to reach any conclusion as i am not able to grasp the logic used to display numbers.