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