I don't think its a buff exactly...
Here is how I think it works:

My gladiator is R50... so is my Lancer

If I want my Gladiator to use "Twisting Vice", an R42 Lancer weaponskill, then it will be less effective
But....
If I use "Twisting Vice" in a battle regimen and put it directly after someone else's Lancer weaponskill, then my "Twisting Vice" maintains the class affinity as if I was a lancer and therefore does full damage

It's similar to stacking different class weaponskills to increase damage... its not a buff, per se.... it just increases the damage of consecutive skills