Use the manual event button as explained by Nath, that will put the file in the RO ( Read Only ) folder.
That folder will eventually also overwrite the old stuff if full, but with a sensible size memory card ( 128 GB at least for a system like that ) and nothing other spamming that RO folder it will be there for weeks / months.
I use that approach myself and only retrieve footage from the RO folder once a month and i only copy the files, and i have yet to see that RO folder recycle, though in all fairness i do use a 256 GB memory card.
but i only generate 5 - 10 manual events a month, and thats barely making a dent in the RO folder, and my SG system save much more than the A129 duo do as the SG cameras save two segments ( 3 minute in my case ) for every press of the button, and then as it is a dual channel camera thats X2, so i get 4 X 3 minutes saved with a press of the event button, the viofo as i recall only save the current file being recorded, in that case if thats 3 minutes too it will just put 2 X 3 minutes in the RO folder,,,, one file from each camera.
I think it might be a dynamic sized RO folder, so if it is hit hard it can also grow larger, but you should not use things that would do that,,,,, like having G sensor on while driving.
Play around with your system and see what you get, fake some events.
I have made it a habit to say why i press the button when i do so, cuz weeks later it can be hard to recall why, so the audio track will tell.
This could for example be ( press button ) wait a few seconds to have clear audio for upload, and then say for instance "idiot in red corolla run red light"
