To "press" a button often implicitly means to push it down and then release it. If I instruct you to "press" the A key on your keyboard, you'll probably push it down and then let go.
To "depress" a button unambiguously refers only to the act of pushing it down.
Consequently, if you need to describe a series of actions involving buttons in unusually intricate detail, it can be useful to use "depress". For instance, it could be problematic I ask you to:
- Press the A button, then
- Right-click, then
- Release the A button
because you may have assumed you were meant to release the A button in step 1, and only realised your mistake when you got to step 3. On the other hand, if I say:
- Depress the A button, then
- Right-click, then
- Release the A button
then the ambiguity in step 1 is eliminated. This makes the word "depress" useful when talking about the act of depressing a button.
There's less ambiguity between the states of being "pressed" or "depressed" than there is between the acts; it's clear that both mean the same thing as long as it's clear that the word is being used to refer to a state. However, there are still at least a couple of reasons that a writer might still sometimes prefer to use "depressed" to refer to the state of a button being held down.
One such reason is to retain consistency with how they refer to the act of pushing down a button, for which they may elsewhere use the word "depress".
Another reason is to reduce ambiguity with the act of pressing the button. Note that the grammar of a sentence will often not convey whether the word "pressed" refers to an act or a state. For instance, consider a sentence like:
The green LED should remain on as long as the button is depressed.
Here, the most natural interpretation is that "depressed" refers to a state; in other words, that as long as I continue to hold down the button, the LED should remain on. But if I said:
The green LED should remain on as long as the button is pressed.
then a possible alternative interpretation becomes more salient, where "pressed" refers to an act; that is, that as long as I perform the act of pressing the button, then the LED should thereafter remain on (even if I release the button).
Thus, if the former meaning is intended, using "depressed" instead of "pressed" here is one way to avoid confusing the reader. (There are, of course, plenty of others; a phrase like "is held down" or "remains pressed" would serve the same purpose as "is depressed" here. But such is the nature of English; there are usually several different ways to say anything.)
Up/Down
complex. The physical pressing down is the basic sense; emotions are metaphorical. – John Lawler May 23 '15 at 20:29