Grid by itself, just monitors health, and gives you the option to target that toon via clicking on their box. I mean the addon can give you so much more information, but health is the main thing.

If you look at the video, he is selecting toons via the Grid, but then pressing his keyboard, which undoubtedly broadcasts the heals to the other toons.

You can combine Grid with something like Clique to make the clicking also do heals depending on how you click the Grid; basically make it perform like Healbot. If you were to do this, you'd need to broadcast your mouse clicks as well, and probably have the slaves with the same grid as you. For heal boxing, via Grid, its probably easiest to just use it for targeting. Click the heal on the keyboard with one hand, while selecting who needs the heal with the other via the mouse.