Tahallul is derived from an Arabic root suggesting opening or untying (a knot). Tahallul in Hajj refers to dissolution or ending the state of Ihraam by virtue of which all restrictions of Hajj return lawful. A pilgrim who makes Tahallul is not in the state of Ihraam and is no longer performing Hajj. 
During the months of Hajj, it is permissible for anyone who enters into the state of Ihram for Hajj to perform Tahallul after making ‘Umrah, and likewise anyone who enters into the state of Ihram for Qiran Hajj may perform Tahallul after making ‘Umrah, if they do not bring Hady. This was authentically reported from the Messenger of Allah (ﷺ). By doing so, they will be falling under the same ruling as a pilgrim performing Tamattu` Hajj. It is not permissible for the Muhrim to remove anything from their skin, hair, or nails until they perform the minor Tahallul (partial ending of the ritual state for Hajj or ‘Umrah through shaving or shortening the hair and throwing pebbles at Jamarat-ul-‘Aqabah before performing Tawaf-ul-Ifadah.
It is prescribed for the Mutamatti‘ (pilgrim performing Tamattu‘ Hajj) who performed Tahallul after ‘Umrah to enter into the state of Ihram for Hajj on the Day of Tarwiyah (8th of Dhul-Hijjah) from the place where they are staying, whether they are inside or outside Makkah or in Mina. The Prophet (ﷺ) commanded his Sahabah who ended their `Umrah to enter into the state of Ihram for Hajj on the Day of Tarwiyah from their houses. Any pilgrim who is prevented from completing the rituals of Hajj should not make Tahallul unless they slaughter a Hady and shave (in case of men) or cut their hair short.
It was reported that: In the Farewell Hajj, the Prophet (peace be upon him) ordered his Sahabah to make Tahallul and make their intention for `Umrah except those who had brought Hady with them.Sahih al-Bukhari 1785, 1651, 4396 and Sahih Muslim 1245