The 400LX is one of the last Shimano derailers to feature an overload spring in the cable anchor. This was a neat feature that allows easy downshifting even if you accidentally stop in a high gear. Just yank the shift lever back to the desired gear, and the cable anchor arm will move as much as needed. Then, as soon as the cranks turn, it will shift to the gear you pre-selected.
I can't imagine why Shimano stopped offering this very nice feature.