Desperation dice

[avram]

A dice mechanic looking for a game:

Assume an RPG where you roll a die, or dice, of varying size to check for success. Like, the dice range from d4 to d12, and you’re looking to get a high number or sum. I think the Cortex system uses something like this — roll a few dice, keep the best two, add them.

So, what if there’s a mechanic that lets you, after you’ve rolled, take out one die and replace it with a re-rolled die one size smaller? Like, you’re rolling a d10, but it comes up crappy (you rolled like a 1 or a 2 or something), so you do a thing (invoke an Aspect, spend a Plot Point, whatever) and you get to yank that die and re-roll with a d8. Odds are pretty good you’ll beat a 2, but not as good as with a d10. This could represent some kind of desperation, a gamble against ever-worsening odds.

I dunno, though. It’s still missing something. I mean, if you’ve got any sense of probabilities, you know how to bet in that re-roll, right? The average roll on a d8 is 4.5, so it only really pays to re-roll that d10 if it comes out 3 or lower. Does this have any teeth to it?