Yoga Sutras 1.31
Patanjali, explained by Swami Vivekananda
दुःख-दौर्मनस्याङ्गमेजयत्व-श्वासप्रश्वासा विक्षेपसहभुवः ॥
duḥkha-daurmanasyāṅgamejayatva-śvāsapraśvāsā vikṣepasahabhuvaḥ
Grief, mental distress, tremor of the body, irregular breathing, accompany non-retention of concentration.
In plain words: When concentration slips away, it comes with sadness, mental upset, shaking of the body, and uneven breathing. The teacher says not to worry about the shaking and to keep practicing, with Om and surrender to God.
Swami Vivekananda explains
Concentration will bring perfect repose to mind and body every time it is practised. When the practice has been misdirected, or not enough controlled, these disturbances come. Repetition of Om and self-surrender to the Lord will strengthen the mind, and bring fresh energy. The nervous shakings will come to almost everyone. Do not mind them at all, but keep on practising. Practice will cure them, and make the seat firm.