OP is talking about pouring from asymmetrical bottles, like a quart of motor oil. Having the opening of the bottle lower than the level of the fluid while pouring causes the fluid to "glug" because of differences in pressure due to the fluid completely blocking the opening, preventing air from entering the bottle to equalize the pressure. Pouring with the opening above the level of the fluid prevents the "glugging" by leaving space for air to flow into the bottle.
redgator12 t1_iy0o4hw wrote
Reply to LPT pour liquids from cartons with the opening at the top and closer to you, rather than directly over your container from underneath. Doing it this way allows airflow and a smooth pour and you won't end up with an inconsistent flow and a potentially messy spill. by basementdiplomat
OP is talking about pouring from asymmetrical bottles, like a quart of motor oil. Having the opening of the bottle lower than the level of the fluid while pouring causes the fluid to "glug" because of differences in pressure due to the fluid completely blocking the opening, preventing air from entering the bottle to equalize the pressure. Pouring with the opening above the level of the fluid prevents the "glugging" by leaving space for air to flow into the bottle.
Video demonstration