To sum it up, when applied to matter of doing work dictates the amount of work done successfully generates as much pleasure and willingness to do more work. This means if you do little work before tackling the big one that you've been procrastinating from then you might as well keep procrastinating. Whereas a succession of work accomplished throughout your day will give you the initial push to get past the initial hurdle of the attempt to do work that is considered boring or challenging.
I say it is practically a law because I always seem to experience the trouble trying to get past the initial hurdle of attempting challenging work, such as math or writing a paper. Getting other work done around that problem gives me enough momentum to push myself to attempt the problem in a different way or slower rather than look at the work as something I want to immediately get out of the way. Writing a bunch of ideas and then outlining is work that you can do before you attempt to write a paper. It builds interest and pushes you to keep going as long as you have the momentum.
Sometimes it is easier to get to work and get into the flow state. I've described this somewhat on a Facebook note. Getting into the habit of doing work instead of lying around or plainly doing nothing. It works sometimes, but maybe that's because there is nothing else to do.
Imagine getting a very hard math problem as the first question on a test. What is your first reaction? "OMG, this test is going to be hard" seems like a valid reaction. Fear has already been instilled and you are on the path to failure. How about if you were to answer the question somehow and then move to the next question only to see it was easier and you were reminded of how to do the first question in the process. Congrats, you just built momentum to get yourself through the test and a road to success.