It doesn't actually save footage. What it does is move 10 minutes of footage prior to honking from the RecentClips folder to the SavedClips folder. If there was no footage in your RecentClips folder when you honked, then it would move no footage. If there was only 5 seconds of footage it would move 5 seconds.
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The way dashcam works is that it continuously records 1 minute (actually 59 second) clips into RecentClips folder. It deletes clips older than 1 hour. Pressing the horn or icon to save clips actually only moves 10 minutes of clips to the SavedClips folder. All this will become obvious if you take your drive/card out of your car and look at what is saved on a computer.
I've seen many accounts where the action of saving footage (whether from horn or other means) means that clip corrupts or part of it is skipped. This has caused many cases where the most critical moment is gone. As such it benefits to only press the horn (or press save) after about a minute since the event passes.