Why is the effect of meditative concentration better the tighter the legs are crossed?

Generally speaking, when sitting in meditation, the double lotus position produces a better effect of meditative concentration (samādhi) than the single lotus position, the single lotus position is better than sitting with legs loosely crossed, and sitting with legs loosely crossed is better than sitting with legs stretched out. This is because when the legs are crossed, most of the qi and blood in the legs gathers in the upper half of the body. With the qi and blood concentrated, the energy is focused, its circulation becomes smooth and forceful, and only then are the meridians throughout the body unobstructed, making it easy to adjust the body well. And once the body is well adjusted, the mind becomes comfortable and joyful, thoughts become concentrated, and meditative concentration is attained quickly and deeply. With the legs stretched out, however, such an effect is not easily achieved: when the legs are extended, a portion of the qi and blood is distributed in the legs, the force of its circulation is relatively dispersed and lacks strength, the meridians are not easily opened, the body is not fully adjusted, and naturally the mind is not so peaceful. 

Then, when sitting with legs crossed, is there no circulation of qi and blood in the legs, leading to paralysis? Of course not. When the legs are crossed, a portion of the qi and blood still circulates in the legs to keep them functioning normally. Moreover, when the qi and blood circulate in the upper half of the body, they also flow toward the legs; with a strong force of circulation, the meridians in the legs are easily unblocked, leg ailments are expelled, and meditative concentration is also attained quickly and deeply.