وَمَنْ لَمْ يَجِدْ ثَوْبًا صَلَّى عُرْيَانًا قَاعِدًا يُومِئُ بِالرُّكُوعِ وَالسُّجُودِ، فَإِنْ صَلَّى قَائِمًا أَجْزَأَهُ، وَالْأَوَّلُ أَفْضَلُ
Translation: Whoever finds no garment prays unclothed while sitting and indicating bowing and prostration; if he prays standing, it suffices, but the first method is preferable.
This applies to an extreme situation where no garment is available. The text permits both sitting with gestures and standing, while declaring the seated form preferable.
Yumi'u means indicating ruku' and sujud; the prostration gesture is deeper than the bowing gesture. The preference for sitting reflects greater concealment.