The spiral lead in will ramp down into the stock using the spiral angle specified.
The angle is the angle of attack relative to 0 degrees = horizontal. So 45 would spiral in on a 45 degree angle. 90 degrees is a vertical plunge. I would recommend using a gentle spiral angle 2-5 degrees.
When it has reached the depth increment depth it will cut a complete circuit then it needs to lift up to move across to the next spiral lead in start. This is what causes that dance and yes it can leave a bit of a tool mark.
In version 0.9.8, it is now possible to specify a 0 degree spiral ramp angle.
CamBam will then use the depth increment to calculate the angle so you get one continuous cut spiralling all the way to the target depth with an extra circuit at the target depth.
As this avoids the safe Z moves it will be faster and leave less tool marks.
At the end of the cut, rather than have the cutter lift straight up (which will leave another toolmark), you can add a lead out move.
This is where the tangent move is useful, so you just need a radius and leave the angle as 0.
I have attached a simple example of this. This will only work in the new 0.9.8 version.