OK this is a pretty old question and I don't know any syllabus that still teaches this but here's my best guess.

So you know that gasses with lower Mr diffuse faster. Hydrogen has lower Mr than normal gas, which contains oxygen and Nitrogen, so it diffuses into the pot faster than air diffuses out.

So for a period of time, there is more gas inside the pot than usual, thus increasing pressure and pushing X down.

After a while, air will diffuse out in order ot reach the same pressure as the atmosphere, since the whole pot and jar is exposed to the normal atmosphere. So the pressure returns to normal and the liquid returns to X

The lower the molecular mass, the faster the diffusion rate, hence H2 (Mr = 2) diffuses into the pot faster, compared to air (approx Mr = 28.56) exiting. This results in a net increase in gas pressure inside, causing the liquid level to drop. The max pressure is when the conc of H2 gas inside and outside the pot is the same. The liquid level then rises as air continues to diffuse out of the pot, until the inside of the pot is at atmospheric pressure. Hence, the reverse occurs if you had started with H2 inside the pot - liquid level rises, then falls back to the orginal level over time.