SB 6.18.61

labdhvā tad-antaraṁ śakro
diteḥ praviṣṭa udaraṁ
 yogeśo yoga-māyayā
labdhvā — finding; tat-antaram — after that; śakraḥ — Indra; nidrā — by sleep; apahṛta-cetasaḥ — unconscious; diteḥ — of Diti; praviṣṭaḥ — entered; udaram — the womb; yoga-īśaḥ — the master of yoga; yoga — of yogic perfections; māyayā — by the power.
Finding this fault, Indra, who has all the mystic powers [the yoga-siddhis such as aṇimā and laghimā], entered Diti’s womb while she was unconscious, being fast asleep.

A perfectly successful yogī is expert in eight kinds of perfection. By one of them, called aṇimā-siddhi, he can become smaller than an atom, and in that state he can enter anywhere. With this yogic power, Indra entered the womb of Diti while she was pregnant.