Let me dig up a picture...
The method works good to keep your rifle on your pack. I use the top compression strap wrapped around the fore-end of the stock to keep it tight to the pack.
Two things that I don't like about the rifle in the grab-it: it's not very quick to access, and depending on how full your pack is, I've noticed that the top compression strap will work itself loose if it is at the wrong angled coming off the rifle to the buckle.
One other thing I'll mention on the grab-it. I keep the bottom two attachment points connected all the time. I simply roll the top straps into the grab it and tuck the whole piece under the sleeping bag compartment compression straps. Keeps it easy-access and you never have to worry about the three bar sliders being difficult in cold weather or with a wet pack!
Mike