Best way to describe it is that you return their damage back to them. You should be constantly weaving in and out of the frontlines. You see your teammate in the middle of the firefight, bubble them. In a middle of a firefight, bubble, and then start wrecking ball until your HP runs down and then immediately duck out until you get your bubble and HP shields back. Remember that half of her HP recovers because of shield. Lob grenades from long range and then when they are close melt them, but if they are in a group grenades work best. Always better as a second tank, hard to work with as solo tank, though I find that she pairs well with reaper.
I run super hot and cold with her though. I can be carving a warpath with her, or I could be only getting like 5 elims with 15% average energy.