Asanas are the means of accomplishing the duties: it is not enough that the yoke is borne, the work must be done. Sana means the presentation, or offering, the gain or acquisition of effort: when an A-sana is done, there is nothing left to gain or acquire, there is nothing to present or offer; one has reached, one has accomplished. Like an arrow discharged, there is nothing left for the archer to do: so it is, that when an Asana is performed, there is nothing left for the Yogi to do. One can sit down.
Anyone can become an acrobat, but it is different to become a Yogi. Sit at your desk at work, sit on the bus, sit in your car in traffic. Sit at the dinner table, and sit on the toilet. Sit on the curbside with your neighbor, sit on the battlefield in victory. Your arrow will hit the mark.