Thought Tokyo stock prices could fall no further? Think again. Corporate pension managers are poised to sell between 2 trillion yen and 3 trillion yen worth of stocks in the coming months -- and think tanks estimate that figure could double.

The prospect of a massive selloff is terrifying business lobby groups and policymakers. Fearing that stock falls could seriously destabilize an already weak economy, they have proposed changes to corporate pension fund rules in a desperate bid to keep stock prices stable.

The proposal presents a tricky maze of pros and cons. Here are some basic facts to help make sense of the problems involved: