It means all spells are affected as each of them got individual cool-down. Example:
You are using Hell’s Core with:
Primary Group attack 4 seconds
Secondary Group Attack 40 seconds
And Individual Group attack 40 seconds
After momentum cool-downs will look like :
Primary Group attack 4 seconds (remains same)
Secondary Group Attack 38 seconds
And Individual Group attack 38 seconds
Another Example for spell Ignite:
Group cool down 2 seconds
Individual cool down 30 seconds
After momentum:
Group cool-down 2 seconds
Individual cool Dow 28 seconds
But it seems it will be useless with strike spells like: Ice Strike and it will not help you with most of Support spells apart: Expose Weakness, Sharp Shooter, Swift foot, Blood Rage, Divine Dazzle, Sap Strenght, Summons, Protector, Conjure Wand of Darkness.
It is designed for strong spells.