Xuesheng, the essence of Yiquan is, you guessed it, Yi. Intention. If you read the PDF, you'll get a better idea of what the student is asked to do during standing practice.
Now, keep in mind, that these intentions can also be applied to the traditional standing postures of other arts with varying effects. The student is encouraged to experiment. It's this internal focus and aforementioned intention that makes this practice more than simply standing.
Along those lines, I've read that the character for sung/song relates to long hair, previously bound up and tied, now let loose and unbound.PS the chinese word 'Sung' in Yiquan (at least for our school) is translated as 'unbound', not just sunk and relaxed, but in a middle place, balanced and free to move without constriction.