Incinerate and magnitude

[Fergard]

Can someone explain in detail what the magnitude is and how it works?
If I want to build through Incinerate and ignite, should I max the magnitude on the tree?
Incinerate itself imposes Fire Exposure stacks, flamability reduces resistances, some tree nodes add resistance penetration and ignite force, but what the magnitude does and what it sums up or multiplies is not entirely clear.
Is this a multiplier? Can magnitudes reduce the target's resistance to less than zero, etc?

[Dentin]

Simply put, higher magnitude, higher DoT for the ailment.

Is it worth it? I mean if making a DoT focussed build with fire...yeah...deff it is. If like my titan and just a supplemental effect but going more for DPS, increasing two handed weapon dmg and armour, eh...it wont hurt to have increased mag, but its not lynchpin of my build either right.