瑜伽論記 4.386

4.386

觀識流轉者，釋別觀察斷，即後三觀，謂觀識斷與未斷，觀察此識生滅相續是極句，或觀生身或觀剎那二種識轉，此上總釋觀察斷義。

Regarding 'contemplating the transmigratory flow of consciousness': this explains the specific 'observation of severance', which refers to the latter three contemplations; that is, contemplating whether consciousness is severed or not yet severed. Observing this consciousness's arising, ceasing, and continuity is the culmination. Either one contemplates the physical body or contemplates the two kinds of consciousness-evolution in a single kṣaṇa. The above generally explains the meaning of 'observation of severance'.