What is the reason why an accelerometer shows -1g value when it's flips?

I was investigating this topic and I thought that the accelerometer shows a negative value (-1g) when it flips due to the gravity, but later I see this video and I got totally confused

In the minute 9:40 to 9:60 it was said that the real reason because of the accelerometer shows -1g its the force of reaction of whatever thing that holds the device. I don’t catch this idea, I mean, I understand that the reaction force of what is holding the device allows measure the acceleration of gravity that suffers the mass (that’s the reason for if the accelerometer suffer a free fall don’t measure anything since that don’t have some reaction force) but I don’t understand how this force comes to play when the accelerometer flips

Let me know if I’m not being clear.

The accelerometer actually measures the difference between the forces applied to the internal sensor (think of it as a mass hanging from a spring) and the sensor mounting frame, which is the black chip itself.

When the accelerometer is in free fall, the force of gravity is applied equally to the sensor and the chip, so the accelerometer reports zero g.

If the accelerometer is prevented from falling, something is holding the chip up while gravity pulls both the chip and the actual sensor down. 1 g is reported.

Flip the accelerometer over and the sign changes, because the force holding the chip up is coming from the other side. -1 g is reported.

If the two results are not exactly +1 g and -1 g, for all three axes, the accelerometer needs to be calibrated. See https://edwardmallon.wordpress.com/2015/05/22/calibrating-any-compass-or-accelerometer-for-arduino/

jremington:
Flip the accelerometer over and the sign changes, because the force holding the chip up is coming from the other side. -1 g is reported.

I don’t understand so well this idea that you try to explain me, can you give me some image maybe? , for what I imagine, the force holding the chip AND the gravity come from the other side is It?