The u-turn is effectively a screw – as you turn the knob the spring rotates, and the spring rod is fastened in place to the lowers. As the spring rotates it moves up and down the rod, effectively shortening or lengthening the rod. The spring itself doesn’t change length.
Does that make sense? So if the 150mm u-turn spring is wound down to 140mm, it effectively shortens the spring rod that is attached to the lowers.
When you do this you also start to move the rebound shaft up in its travel as its fixed to the lowers.
Hence why you can’t adjust the travel with the fork locked out as it stops the rebound shaft moving up inside the damper leg due to hydraulic lock.
Which takes me back to my original point – if the damper shaft is longer than the spring rod, you may end up stretching the spring out.