Considering this is a physics website, everyone that reads this post will have probably already known what "g-force" is.
But I'm going to talk about it anyway.
Whether you're sitting, standing, or walking while reading this - you're experiencing about +1g. All that means is that you weigh exactly what you weigh. So, if you weigh 150 pounds and you're experiencing +1g, that means you weigh 150 pounds. Simple.
But what if you stood upside down? Are you still under +1g? It sure doesn't feel like it, especially with all the blood in your body rushing to your head.
All in all, you're under 1g. The act of flipping yourself over doesn't change any centripetal force...just your human perception.
However, there are various ways to change your "g" level.
Try a centrifuge, for example.
Used to train pilots and astronauts, centrifuges spin around a fixed radius, creating values of up to 9 or 10g - which can get pretty funny.
A pilot undergoing 9g doesn't look very attractive. Their eyes attempt to disembark from their heads; everything is pulled down at nine times its original weight.
An average 170 pound man, under 9g, weighs a whopping 1530 pounds. The centripetal force results in a pulling action on the subject; blood rushes to the lower extremities, leaving the brain in a severe lack of blood. Which is hilarious to watch, but can be extremely dangerous.
G-force has the ability to kill pilots in the blink of an eye. Called G-LOC (G-force induced loss of consciousness), this event is pretty much a killer. Pilots, either cocky or untrained, sometimes pass out when they're under high G-force. The lack of blood in the brain makes the pilot slowly and steadily black out.
Personally, I've only felt 4g, and even that's uncomfortable.
So how did the aviation community overcome G-LOC?
The answer was in the form of an inflatable suit...commonly known as a "G-Suit". Attached to a pilot's flight suit, the apparatus will inflate around the legs and lower abdomen when high G-force is detected. The compression pushes blood back up to the head, so the pilot retains consciousness.