Why does the tighter you cross your legs, the better the effect of meditation?

Generally speaking, when sitting in meditation, the full lotus position produces better meditative concentration (samādhi) than the half lotus, the half lotus better than sitting cross-legged with both legs loosely folded, and sitting cross-legged better than sitting with legs extended. This is because when the legs are folded, 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 the meridians throughout the body become unobstructed, making it easier to adjust the body properly. Once the body is well adjusted, the mind becomes comfortable and joyful, thoughts become concentrated, and meditative concentration is attained quickly and deeply. Sitting with the legs extended, however, does not easily produce such an effect: with the legs stretched out, a portion of the qi and blood is distributed in the legs, so the force of its circulation is dispersed and lacks strength, making it difficult to open the meridians. The body is then not fully adjusted, and naturally the mind is not so peaceful. 

So when the legs are folded, does the blood circulation in the legs stop entirely, causing paralysis? Of course not. When the legs are folded, some qi and blood still circulates in the legs to keep them functioning normally. Moreover, while the qi and blood circulates in the upper half of the body, it also flows down into the legs; the force of this circulation is strong, which makes it easy to open the meridians in the legs, expel illnesses of the legs, and attain meditative concentration quickly and deeply.