Envelope editing

 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.