Under the hood, Do Less goals are implemented the same way as Do More goals, but on the user side the result looks completely different. I’ll try to explain as best I can:
The fundamental rule is that Beeminder derails your goal if there are two consecutive days in the red. Of course we all know that Do More goals can sit in the red for a day while you work on them. They derail at midnight because at 12:01 AM it’s now two red days in a row. You can prove this by editing old data for a Do More goal. You can make a red goal derail immediately if you retroactively make yesterday red too.
Now the catch with Do Less goals is that you are given your daily “credit” before Beeminder checks whether you have derailed. Suppose your Do Less goal is set to 14 per week or 2 per day. That means that if you end your day at -1, at 12:01 AM you’ll be at 1 (since -1 + 2 = 1) and will not derail. But if you ended the day at -3, you would derail since -3+2=-1 is still red.
There’s one more rule though! You can only use the dip-into-the-red trick on alternating days. So suppose your Do Less goal is at 1 and you go to -1. If you didn’t fudge yesterday, you’ll sit at -1, then go to 1 at 12:01 AM and be okay. But if you fudged yesterday, you’ll derail immediately when you go to -1.
So, in summary, to figure out whether your Do Less goal will derail you need to be keeping track of your daily credit and be very careful to only go negative on alternating days and never by more than the amount of your daily credit.