I'm having difficulty manipulating envelopes.

I open them in an automation track, but what I want to define a section (like 2 bars) and drag it up or Down. But when I go to the top of the track and get the up/down icon and click, it doesn't always just move the section I defined.

It will add two more nodes and create a new one, or select and move a section of the envelpe adjacent to the section I defined.

I usualy define a section with 4 nodes: the outside two to keep the rest of the envelope in place and the inner two define and move with the section I want to move.

But clicking adds more nodes and I can't figure out the logic or the technique.

Can't find anything on this in Help.

Help.

Try holding the ctrl key and clicking on the line segment to move an indivdual segment. 
 
Edit: I forgot that using the line segment tool click above the two nodes which selects them then click drag on one of them to move the segment
 
HTH.