The austerities taken by Amba is described in Mahabharata, Udyoga Parva, SECTION CLXXXIX as:
Without food, emaciated, dry, with matted-locks and begrimed with filth, for six months she lived on air only, and stood unmoved like a street-post. And that lady, possessed of wealth of asceticism, foregoing all food in consequence of the fast she kept, passed a whole year after this, standing in the waters of the Yamuna. Endued with great wrath, she passed the next whole year standing on her front toes and having eaten only one fallen leaf (of a tree). And thus for twelve years, she made the heavens hot by her austerities. And though dissuaded by her relatives, she could not by any means be weaned off (from that course of action). She then went unto Vatsabhumi resorted to by the Siddhas and Charanas, and which was the retreat of high-souled ascetics of pious deeds. Bathing frequently in the sacred waters of that retreat, the princess of Kasi roamed about according to her will. Proceeding next (one after another) to the asylum, O king, of Narada, and to the auspicious asylum of Uluka and to that of Chyavana, and to the spot sacred to Brahmana, and to Prayaga the sacrificial platform of the gods, and to that forest sacred to the gods, and to Bhogawati, and, O monarch, to the asylum of Kusika's son (Viswamitra), and to the asylum of Mandavya, and also to the asylum of Dwilipa, and to Ramhrada, and, O Kaurava, to the asylum of Garga, the princess of Kasi, O king, performed ablutions in the sacred waters of all these, observing all the while the most difficult of vows.
After this Lord Shiva appeared before her and gave the desired boon. It is described in Mahabharata, Udyoga Parva, SECTION CXC.
The god of gods, having the bull for his symbol, then said unto that maiden, 'The words I have uttered cannot be false. O blessed lady, true they will be. Thou shalt slay Bhishma, and even obtain manhood. Thou shalt also remember all the incidents (of this life) even when thou shalt obtain a new body. Born in the race of Drupada, thou shalt become a Maharatha. Quick in the use of weapons and a fierce warrior, thou shalt be well-skilled in battle. O blessed lady, all that I have said will be true. Thou shalt become a man at the expiration of sometime (from thy birth)!' Having said so, the god of gods, called also Kapardin, having the bull for his symbol, disappeared then and there, in the very sight of those Brahmanas.
