Are you ever in the middle of explaining something, something you thought you'd figured out, when your explanation falls to bits? Where the joins between your points look shaky and you start to doubt the points themselves?

Don't beat yourself up. It happens a lot, and to all sorts of writers. For example, you might think that judges never doubt their reasoning. But, when writing their formal "opinions", they experience this as much as anyone. In fact, they coined a term for it. According to legal professor and attorney Kathleen Waits, they:

…frequently discover that the original opinion they originally had in mind 'won't write'.
– "Values, Intuitions, and Opinion Writing", Waits, 1993

"It won't write" means that even when you produce some words, they no longer make sense. US Court of Appeals Judge Richard Posner wrote that:

…Reasoning that seemed sound 'in the head' may seem half-baked when written down…
– "Judges' Writing Styles (and Do They Matter?)", Posner, 1995

Now, you could just take this as "legal folk get writer's block like anyone else". But you'd be missing something important. Judges, at least some more self-aware ones, take this as a chance to improve their decisions. Professor of Law Mathilde Cohen provides more context:

…reason-giving imposes a form of self-discipline that is thought to improve the quality of the decisions themselves. This process is often described as the 'it won't write' phenomenon. In attempting to reason her decision, a judge discovers that she cannot find an appropriate legal justification, leading her to reconsider her initial ruling and make a more accurate determination.
– "When Judges Have Reasons Not to Give Reasons", Cohen, 2015
Professor Mathilde Cohen. (Image from BU Center for the Study of Europe.)

So by recognizing the signs, judges know to rethink their initial decisions. You might wonder whether this improvement is real or illusory. You might also ask, "Even if it's real, does it only work when writing opinions before judgement?", You might think perhaps these judges should just discuss more together to get the same effect.

The research literature suggests not. It suggests that there's something special about writing opinions before finalizing them; something that uniquely helps the writers see objectively and make judgments with less bias. Judge Patricia Wald wrote that:

Even when judges agree on a proposed result after reading briefs and hearing argument, the true test comes when the writing judge reasons it out on paper (or on computer). That process more than the vote at converence or the coutroom dialogue, puts the writer on the line, reminds her with each tap of the key that she will be held responsible for the logic and persuasiveness of the reasoning and its implications…
– "The Rhetoric of Results and the Results of Rhetoric", Wald, 1995

And with cognitive psychology, John Zhuang Liu found in his 1998 study "Does Reason Writing Reduce Decision Bias?" that judges were indeed less biased when writing judicial reasons before concluding a decision.

But do we need all this research to tell us that writing can help us test our reasoning? Maybe not. It's an effect that's so direct, so clear when it happens, that it doesn't depend on external proof. It seems to work for all kinds of explanations and arguments, whatever the technical or organizational domain. If you're not sure, just try it, next time you have to explain something complex. Write your explanation in sentences and paragraphs, not bullet points (here's why), and see how far you get. If you get stuck because you realize you didn't understand the concept as well as you thought, or a connection doesn't hang together, there's your "it won't write" effect. Use it to rethink what you're writing, and your explanation will improve.

But there are some people who don't experience this so much when they write. They rarely find that their reasoning "won't write"; words flow directly from an idea in the mind to an argument in text. I think this happens for one of two reasons: one better, one worse:

Long experience makes writing flow easily. Some folks's experience is so ingrained that they don't stumble when writing mature, well-connected explanations for things in their domain. That certainly speeds things up, though could these writer's experience have become fossilized? Are they as open to seeing gaps or other opportunities as they once were? If the most senior judges still appreciate that "it won't write" from time to time, surely it's worth continuing to challenge ourselves.

Writing flows because it's a collage of ideas, poorly connected. More commonly, writers recycle other people's words and ideas without thinking them through fully; without really owning them. It could be industry buzzwords or academic jargon. Often, this language is full of undigested nouns and noun phrases (verbs help puncture such unexamined ideas, by making them rub up against each other, forcing them to connect meaningfully or make way for ideas that do). This unthinking recycling is a hard habit to break out of. Judge Posner has some advice. The "impure" writing style he suggests, that helps fresh thinking reads like talking to another person, avoids:

…the 'professionalizing' devices of the purist writer–the jargon, the solemnity, the high sheen, the impersonality, the piled-up details conveying an attitude of scrupulous exactness, the fondness for truisms, the unembarrassed repetition of obvious propositions, the long quotations from previous cases to demonstrate fidelity to precedent, the euphemisms, and the exaggerated confidence…
– "Judges' Writing Styles (And Do They Matter?)", Posner, 1995

Though even Posner himself isn't so easy to read now, the point's clear: treat writing as a conversation, and don't use language to cover up undigested ideas. Like this, you can write to decide, not decide and then justify.  

So, blocks in your writing can be really useful. That is, if your writing process makes you realise that you could make a better argument or explain more clearly.

Which raises another question – what's the difference between these temporary "writer's blocks" that show the gaps in your thinking, and other kinds of block that might come from struggling for the right words, or being intimidated by the audience or the process of writing itself?

To me, these two kinds of blocks feel different. The "it won't write" block is like a cold clear light: you see where you've gone wrong, even though you don't yet know how to fix it. The second kind of block is less clear, needing digesting and patience of a different kind. Neither is fun, but perhaps I appreciate the "it won't write" kind more. I don't worry as much now when I see a big hole in my logic. It's a chance to improve, to get across a point more clearly or even make a different, better point.