And he shall burn the heifer before his eyes; its skin, its flesh, and its blood, with its dung, he shall burn.
Torah Temimah: Yoma 42a — What is “le’einav”? That he should not divert his attention from it.
English: What is “le’einav”? That he should not divert his attention from it.
Definition of “le’einav” as continuous intentional focus.
The Kohen who oversees the burning must maintain unbroken attention on the animal throughout the process; diverting his mind invalidates.