"An implicit understanding is anything a skilled geek would have to be told before being set loose in your codebase to make a change. Implicit understandings take lots of different forms, but at their simplest, they often involve hidden correlations."
Computers are very literal. They always do exactly what you tell them to do. Even (especially?) when that's not what you want them to do. And yet we often write code that lets us do that.
How many times have you come across a library that requires you to create an object, then use that object to set some properties before you can actually use it? It's a fairly common pattern. And it's an example of implicit requirements.
You need to know that before you can use the object you need to initialize it. That's not an unreasonable thing, but why write a library that makes the user have to remember that. Consider an HTTP request class. It probably has a member that determines if it's a PUT, GET, PATCH, DELETE, etc. And the typical use case looks something like
req = new HttpRequest() req.Method = HTTP.GET req.URL = "someurl" . . . resp = client.Do(req)
That works, but it's possible, and if the code is more complex, easy to not set the Method or URL members. And then at runtime you get some kind of error. So why subject yourself to that kind of delayed error?
You can prevent that kind of error by being explicit. Instead of creating a bare Http request, explicitly create a GET request, and make the creator require a URL. Something like
req = new HttpGetRequest("someurl") . . . resp = client.Do(req)
With that pattern it's impossible to execute a GET request without setting a URL.
So next time you're explaining a feature or bug fix and part of the explanation includes the line "And before you do X, don't forget to do Y", take a look at the code and see if you can turn an implicit requirement into explicit code.
For more examples of implicit requirements and explicit code to remove them, check out this thread.