what is meant by downward displacement of water
In downward displacement of water, you immerse a jar in a large container of water and let it fill completely with water. Then turn the jar upside down, and put the tube for the gas under the jar opening. The gas enters the jar and forces the water down and out as the jar fills with gas - so it it called "downward displacement of water." This is a good way to collect a jar of oxygen, hydrogen or natural gas from the gas tap.