Here's how the $999999 trick works...
The automatic tax screen comes up and attempts to ensure that the maintenance percentages will not take more money than you have.
The actual effects of maintenance will only happen one "clock tick" later, after you close the window. Holding any button prevents this "clock tick" from passing.
The usual method, at this point, would have too little money to pay for maintenance and the auto-tax reduces the maintenance percents to fit. So it manually goes back into the tax screen to raise the maintenance again. This manually opened tax screen has no such safeguards in place to stop you from raising the maintenance.
The method used here instead has enough money to pay for maintenance, so the auto-tax doesn't mess with your maintenance. However, while making sure that next "clock tick" doesn't happen, spend more money until you can't normally pay the maintenance. Now let time pass.
Since the maintenance assumes the auto-tax did its job (
a good assumption only if the player couldn't affect maintenance or money between these events), it takes your money, whether or not you have it. Thus, it sends the money to negative, then the game thinks it's a really big positive value, and clamps it to $999999.
Someone wanted the trick explained, there you go. It's my interpretation, anyway. After all,
I knew of the new method a few months ago.
As someone who attempted to TAS this game, but lost motivation to do so... I will withhold my opinion unless someone specifically asks me to give it. It is not good, I will say that much for now.