Originally Posted by
dstrong
The inward/outward movement of the derailler is controlled by the derailler cable, not the chain. The chain breaking wouldn't cause the derailler to move in or out.
I think you'll find that there is a spring in the derailleur that places it under tension toward the smallest cogs. Thus, an increase in shifter cable tension will work against the spring and shift the derailleur inwards, whilst a decrease in tension will do the opposite.
This also explains why shimano STI shifters have the downshifts on the brake lever - to get more leverage against the derailleur spring tension.