Warp Tension Control and the Moveable Front Beam

As one presses down on a treadle to open the shed, the breast beam angles to  decrease the tension on the warp, so that even with an inelastic warp like linen the treadling is light.
With a metal wire warp, inelastic, a shed is only possible if the distance between the breast beam and the back beam becomes smaller. When the shed is made wider,  and the tension on the yarns increases, the beam moving allows the shed to form with little increase in the treadling pressure. 
When you first set up your warp and have the tension that you want, set the tension on the spring so that the floating arms are vertical by tightening or loosening the cord.  Every time you move the warp forward, you then wind on till the floating arms are vertical
Advantages of the Moveable breast beam
Makes treadling light with inelastic yarns
Helps to make a shed, as the end of the warp nears the shafts.