A type of Nāma (body-making) Karma, by the Udaya (rise) of which the Jīva (soul) gets the capacity of movement in desired direction. It becomes the cause for taking birth in two-sensed beings and so on.
trasanti- uṣṇādyabhitaptāḥ santo vivakṣitasthānādndvijante gacchanti ca chāyādyāsevanārthaṃ sthānāntaramiti.
trasā-dvīndriyādayastatparyāyapariṇativedyam nāmakarmaṇi trasanāma.
(Prajñā 23.38 Vṛ Pa 474)