There's too much going on... but not right now.
My great colleague, Tony Chemero, studies human-tool interaction. This includes studying how objects like a blind person's stick, a musical instrument, or a mouse become 'invisible' to their user. You know the feeling. If you are reading this post, you got to it by clicking on a link. When you did that, you didn't first think "I'm gonna move my [say] mouse so that the cursor is over the link, then I'm gonna use my index finger to press down. Here I go..." Rather, you thought "I want to click on this link" or perhaps just "I want to read this post" and your fingers did the rest. Moving the [say] mouse was as scratching your head.
And you definitely know the feeling when a tool is not invisible. Much of the time, working on a computer is like that. Computers offer so many possibilities for entertainment, procrastination, and gawkery that it is very easy to lose one's focus. When that happens, you need to think, very consciously, "What was I just doing? What was I meaning to do? What do I need to do to get back to it?" Add to that the frustration of badly designed software, and it's hard to get anything done on a computer.
One thing that helps is to artificially narrow the possibilities a computer offers [what it 'affords,' to those following Tony's work (after J. J. Gibson); and thus the title of this post, which is borrowed from Erik Rietveld]. The application Focus does that. It blocks distracting sites and distracting applications. There are lots of programs that can do what Focus does, but I like its particular combination of features.
Focus has two states, focused and unfocused, and thus two transitions, from unfocused to focused and back. Focus lets you decide which applications and which websites it will block in the focused state. You can block, for example, Facebook and the App Store, but allow Wikipedia and Excel. Focus also allows you to run scripts, commands, or open and close applications during transitions. I use it, for example, to open Scrivener when I start to focus and to open my mail program when I finish. Finally, you can set a timer or a schedule for the focus; e.g., tell Focus to focus for an hour, or between 9AM to 10AM, or for the next 25 minutes. Concentrating is hard enough, let the computer narrow its own affordances.