The process of testing a software product may sound simple, but, in fact, it isn't. The one most important rule that developers and testers need to follow is to have a 100% transparency when communicating. Therefore, with problem reporting, testers should provide as much detailed information, or test artefacts, as they are called, as possible. Test artefacts do include logs, screen grabs, time and date, ways to reproduce and a lot more data, but more on those later.
With all of that said, screenshots are certainly some of the most valuable test artefacts, and with Android devices being manufactured and customized software-wise by dozens of different OEMs, capturing ways do vary. As to what are the key combinations preferred by devices with regards to brands, the one that sticks out the most is Samsung and their older models. You all have seen their phones and tablets from the past couple of years and beyond that have hardware home buttons.
Perhaps, with the Galaxy S7 lineup and previous ones, you can capture a screenshot by holding simultaneously the home button and power button. Meanwhile, many Huawei phones incorporate an additional method to the traditional power and volume down key combo that is applicable with most Androids, and it does rely on a technology named Knuckle Sense. As you are guessing, it requires you to use your knuckles, and more precisely, to knock twice on the display. Similarly, OnePlus has added a three-finger swipe gesture, which you can try as well. Other than those, Google's Assistant can be utilized for the same exact purpose. Just ask it to take a screenshot, or tap the "Share screenshot" button above the microphone icon and it will be saved straight to your gallery.
Optionally, instead of all of the ways that we shared above, and in times when your device's hardware buttons do not function properly, you can get an app from the Play Store that has other types of triggers. One popular example would be Screenshot Easy and it has more than 10 million installs already. Also, it lets you draw on screen captures that you take, which is certainly a good practice when explaining graphical problems that you may be facing. Google Pixel phones and many more that are running the latest Android Pie are being shipped with an app named Markup, which does provide an identical feature.
To conclude, we have to agree that in-depth explanations, including screen grabs with drawing on them, are an essential part of the testing artefacts set.