Bhagavad Gita · Sankhya YogaChapter 2, Shloka 58 (2.58)

The tortoise comparison. The moment a tortoise sees danger, it draws its head and feet into its shell; when the danger has passed, it brings them out again. Shankaracharya: yathā kūrmaḥ bhayāt svāny aṅgāni upasaṃharati — just as a tortoise draws in its limbs out of fear, so the seeker engaged in the discipline of knowledge draws in his senses from all objects.

There is a subtle point in this comparison: the tortoise's limbs are not destroyed; they are under its control — it draws them in when it wishes and brings them out when it wishes.

Ramanuja — the importance of timing. Ramanujacharya: yadā indriyāṇi indriyārthān spraṣṭum udyuktāni tadā eva — the moment the senses get ready to touch their objects, at that very time draw them in and fix the mind in the self. That is, not late — right at the start.

The four steps in summary (Ramanuja): this is the fourth and very first step. Here Ramanuja says: evaṃ caturvidhā jñāna-niṣṭhā pūrva-pūrvottarottara-niṣpādyā — from 2.55 to 2.58, four kinds of the discipline of knowledge, in which each higher state is achieved through the one below it.