-You have a 1/100 for a legendary class set to roll super ancient.
-If you have 4 super ancients from the same set, you will gain a "weak" skill rune for the skill(s) that is affected by the class set you have equipped i.e IK set will only give you skill runes of WOTB/COTA.
-If you have 6 super ancients you will gain a "strong" skill rune for your set skill(s).
It would fit in perfectly with all the other RNG systems already in play. Why not add one more.
Personally I'm not a huge fan of it. When it works for you, it makes you feel awesome and super powered, but when it doesn't, you feel like you're behind everyone else who got lucky. Like rolling the perfect Ancient weapon with ideal stats, high base damage and max affix. You either get lucky or your progress gets soft capped. That's what this looks like to me.