The idea of using constraints to your advantage is important to me because it forces me to not care about how something looks, but force myself to focus on how something functions. I also like to call this the less is better rule.
This is Jack White talking about the power of constraints and using them to your advantage. What he's talking about is that all the tools and money in the world can actually prevent you from being creative if you end up thinking about tools more than coming up with the ideas.
The idea also reminds me of an interview I once read. I think it was either an interview with fashion designer Michael Koors or with Tim Gunn. Sadly I can't recall where I read it. But the gist was that fashion guru in question was asked why he wears nothing but black suits. He said something like wearing simple and black means I don’t have to bother with making decisions about what I wear every day, so I can focus on the harder decisions about what other people are going to wear. I love this idea. Barry Schwartz's Paradox of Choice also talks about this idea of removing choices might be one way to help focus on the important problems.
There are lots of ways you can put this guideline to work in how you sketch, or in anything you do as you make products. To put it in simpler terms, I've thought of this as "Less is Better" when it comes to reducing tools and devices in the periphery.
Here is an example of one expression of the idea.
I picked this constraint. To capture ideas:
Forget about the rest, because you're just trying to get the idea from here down to paper.
For you it could be something else. Take the example of Twitter. It's amazing that something as overlooked as imposing a constraint on the number of characters in a message can be so powerful, but the constraint leads to a very different form of communicating and a different way of framing ideas compared to longer, and often less edited messages. The constraint can come from anywhere, but for the technique of sketching, I focussed on the pen I use.