Apropos of nothing I believe this works because it frees up space in your brain. Sometimes, when I'm trying to get too much done at once, I'm stressed out by trying to keep to many things in the forefront of my thoughts at once. When I get stuck like that I create a scratch pad document with three 'zones'
stuff I'm working on right now
Stuff that I know needs to get done
Stuff that is now done.
Start by dumping everything I'm thinking of in 'Todo' and pick one and put it in Doing and while I'm in the middle of doing it when I think thoughts like "oh and this should really do x" I add that to the Todo pile and go back to doing. Each time I finish of the 'Doing' task I scan the todo list, move anything I need to into Done and pull one up for the Doing pane.
By externalizing the bookkeeping of all the things I'm trying to keep straight in my head I free up cycles to actually work on something.
The recently discussed http://mindmup.com works beautifully for me for this -- I just start lobbing the features and head noise into nodes and organize until some watershed where I realize I know what I need to do.