8% of Armour applies to Fire, Cold and Lightning Damage taken from Hits

I got 20452 armour and 79% fire, 78% cold and 79% lightning resistances.

What is the math behind this? How much damage would 10000 fire damage deal to me?

10000*0,21 = 2100 fire damage after resists. That damage then is further reduced by 8% of your armor 20452*0,08=1636 armor applies. Putting both numbers into the armor formula we get: 1636/(1636 + 5x2100) = 0,135 or ~13,5%. Meaning your armor will reduce the 2100 damage by another ~13,5%. 2100x0,865= 1816 fire damage taken.

As with regular armor the effect is lower the higher the incoming damage is but it's powerful against small hits and with more armor it's also very potent against strong ones since they get reduced by resistances first.

If you're using the 8% of armour thing, get your resists as high as you can, since (as mentioned) armour works better against smaller hits, and the resistances are applied first.