They shift the last element attack over to the weapon.
For example, you're fighting as Lynx, against the Red Dragon God. You're weapon attribute is black. You use a water spell, as its most effective against red (duh). Now your weapon attribute is blue, so your weapon is super-effective against the Red Dragon.
Now there's sigils a,b, and c, but I really don't know exactly how much more effective each one is over the last.