labdhva tad-antaram sakro
diteh pravista udaram
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Finding this fault, Indra, who has all the mystic powers [the yoga-siddhis such as anima and laghima], entered Diti’s womb while she was unconscious, being fast asleep.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
A perfectly successful yogi is expert in eight kinds of perfection. By one of them, called anima-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.