Lifetap pertaining to Ressanant Shield

This is the sort of thing that drives me CRAZY in this game.

According to the tooltip, my ressonant strike does .5x the damage as my shield charge.

But even after I sock it with a lifetap gem for 8% extra percent, it does like NO LIFETAP AT ALL!

This is the recurring confusing thing that drives me CRAZY about this game!

Does anyone know why this is? It's a physical attack, it's an attack, and yet it doesn't seem to lifetap AT ALL.

MAKE IT MAKE SENSE.