This idea is born in answer to "100% Chameleon" trick in Morrowind, but has a much wider field of uses.

Imagine, your Player Character can accumulate power-ups (with, for example, Fire Resistance) and wear many of them at the same time. How do you both give these power-ups substantial boost (20, 50, 70%) and at the same time prevent player from combining them and becoming 100% immune to fire?

Of course, you can make it so the stronger one just superseedes the weaker one, totally annulling it, or you can make it as in AD&D series of games, where one cannot wear two items with the same effect...

But there's a better way!
Namely, 50% + 50% = 75%.

In short, internally you keep not the resist/weaknes percent, but a strength factor. I.e. 20% corresponds to 0.8, 75% to 0.25, etc. Now, when you need to perform an *addition* of all these effects, you instead perform the *multiplication* of all the strength factors. Then you multiply the value you got by the fire damage PC receives, but in the interface output you display the total effect in a percent form as 100 * (1 - factor).

This way *all* the power-ups will help to increase the resistance, but the more powerful ones will me much more useful. You'll need no less than *five* 25% power-ups to get 75% in total. And so on.