keeping the default envelope on applied vertices?

I’m attempting to rig up a human for the first time, and in doing so, I notice that when I test the rotation of the parts that i’ve hooked up by setting the vertex groups, all theother parts of the model that I have not assigned groups to, but have parented to the armature, move slightly as well. However, when I assign those portions to the armature, they no longer do this.

Now I’m intelligent enough to do some figuring out on my own, and realise that this behaviour is due to the ‘envelopes’ of the bone. But I cannot figure out how to keep this behaviour. I’d really like to be able to keep this in because it makes the figure seem more natural. The way I have it now, when I rotate her head it ‘warps’ and streaches (I’ve figured out weights, but find it really annoying to estimate weighting behaviour when the default enveloping looks so normal)

If this is possible, how do I edit the envelope size? (so that when I rotate the head it only effects the neck, but not the chest because otherwise it will be incredibly difficult to pose.)

I hope what I described makes sence, I’m figuring this out practically form scratch so I might not be using the right terms hehe :).

Yes you can adjust the envelope size. Go to the envelope mode, and use S to size the envelopes.

http://www.blender.org/cms/Armature_Envelopes.647.0.html

After you’re satistied with envelopes, you can bake them to weights and do the final polishing there.