Pushing a button is more disruptive than looking down, because looking is the default method by which we seek information.
Here's a demonstration - answer this simple question.
What is the exact time right now?
And your eyes just moved to the nearest clock - probably to the lower right of your screen, possibly to your watch or a wall clock. No matter where your eyes go, they go there because your brain maintains a sophisticated map linking a desired piece of knowledge (the time) to the place you can find it. This is a single-step processing "primitive," and it works quickly and instinctively.
Now try this one.
What's the top application in your task manager?
And I'll bet you just skipped past that question and kept reading, because to get the information you have to push Ctrl-Alt-Del to bring up the task manager, visually locate it and then reclose the task manager. Even though it's just a button push and takes only a couple of seconds, doing this task would take you out of the flow of reading in a way that checking the time does not.
In fact, your visual system can lock onto a known-location target (saccade is the technical term) in as little as 10 milliseconds when trained to do so, and the relevant information is in your brain 10 milliseconds after that. Compared to this a two second button-popup-button-close sequence is ten times slower. It's actually much worse than that for a bunch of other brain-processing reasons which I'll spare you unless you really want to know. So pushing a button for information is much more disruptive to whatever you're doing than just glancing to the place it always exists.
However, on a deeper level the issue is NOT pushing the button, else I would push the button once and leave the clipboard up. The issue is that the clipboard is view-locked, and the real world does not have view-locked elements. My wife is an airforce science officer who's currently working on helmet mount systems for military aircraft. Being geeky, I read everything over her shoulder, so I can tell you that view-locked information is used very sparingly in HMD, because it can cause spatial disorientation. It's also always transparent, and focused on infinity (that's why the text buffer is less an issue than the clipboard). Aces High has no IMC, so SD is not an issue, but having the clipboard jitter around in relation to the plane and the world ruins immersion for the same reason view-locked HMD elements cause SD. The brain gets two conflicting references for what the "real" orientation is, and the mental gyros tumble. In a real plane, you crash. In a simulator, you just don't feel you're flying a real plane.
Which is what we're here for, after all!
Hope this makes it clearer.