O King Pariksit, opening her (Putana's) mouth wide and spreading her arms, legs and hair, she fell down in the pasturing ground in her original form as a Raksasi, as Vrtrasura had fallen when killed by the thunderbolt of Indra - SB 10.6.13

Expressions researched:
"O King Parīkṣit, opening her mouth wide and spreading her arms, legs and hair, she fell down in the pasturing ground in her original form as a Rākṣasī, as Vṛtrāsura had fallen when killed by the thunderbolt of Indra"

Srimad-Bhagavatam

SB Canto 10.1 to 10.13

O King Parīkṣit, opening her mouth wide and spreading her arms, legs and hair, she fell down in the pasturing ground in her original form as a Rākṣasī, as Vṛtrāsura had fallen when killed by the thunderbolt of Indra.

SB 10.6.13, Translation and Purport

In this way the demon Pūtanā, very much aggrieved because her breast was being attacked by Kṛṣṇa, lost her life. O King Parīkṣit, opening her mouth wide and spreading her arms, legs and hair, she fell down in the pasturing ground in her original form as a Rākṣasī, as Vṛtrāsura had fallen when killed by the thunderbolt of Indra.

Pūtanā was a great Rākṣasī who knew the art of covering her original form by mystic power, but when she was killed her mystic power could not hide her, and she appeared in her original form.