User blog comment:B214/Volume 17 spoilers./@comment-3965848-20140222084600/@comment-16281343-20140222084933

I think what that means is that he can remotely boost a projectile spell, much like how he can divide.

Issei cannot boost an attack that has already been fired, but maybe now with DWF he can.

For example, he doesn't have to get close to Rias to boost her POD shots. Rias just needs to fire one and Issei will take care of it.