Why a ketchup bottle is turned upside down then thrust downward and stops suddenly?

The ketchup bottle has a Venturi effect in the neck. The narrow neck of the ketchup bottle causes the ketchup to flow faster, which creates a low-pressure area behind the ketchup. This low-pressure area sucks the ketchup out of the bottle. When the bottle is thrust downward, it is suddenly stopped due to inertia, and the ketchup keeps flowing out of the bottle due to the Venturi effect.