The clue is in the name. It's an average of your speed between two points, and on longer stretches can have multiple cameras averaged out together.
If you drive at 90mph through the first checkpoint and 50mph through the second checkpoint you'll have averaged 70mph over your total distance (given equal amounts of 90mph and 50mph driving)
Say you cross the first checkpoint at 50mph, and the last checkpoint at 50mph but it only took you 4 mins to cover the 5 miles of average speed zone - your average speed is 75mph. So despite slamming on the brakes for the cameras, speeding up during that journey will still catch you out.
Most Sat Navs or Google maps directions will tell this for you, some modern cars will too. But the only way to truly be safe is to be aware of your speed across your entire journey.
If you want to do 90mph know that you have to do 30mph at some point to even out your speed. otherwise stick to 50mph and you'll be fine.
SPECS cameras are often installed 200m apart pre-installed to know that it take about 9 seconds to pass from point A to point B to average 50mph. If you 60, you'll need to do 40 at equal time to average out at 50mph.
SPECS 3 cameras can be 75m apart for more accurate readings.
On smart motorways, where the big red circles changes on the overhead gantry, the cameras here will automatically change parameters to adapt to the new limits. This is more obvious for the HADEC 3 cameras that will flash you, but also applies to some average checks.
Yes, 24/7 they use infrared and ANPR (automatic number plate recognition) to track and flag cars all year round without the need for flash.
Driving the speed limit helps, although there may be a few mph tolerance on most camera, it's unwise to test the limits.
Have you been paying attention? You can't 'beat the system' if you pass between point A and point B you need to average 50mph or less to not be fined. If you've been averaging 50mph on your speedo the entire distance and by the end you've crept up to 55mph without realising, you'll still average within the limits over the total distance. Don't cause an accident by hitting your brakes at the end for the cameras.