Nage means the last phase of a throwing technique. Some argue that the phases of a throw are completed with Kake (throw execution). However, in this last phase of a throwing technique an action of TORI is needed, because he is responsible to minimize the risk of injury to UKE. (Judoprinciples).
Kake is excreted in Nage after TORI has performed the intended technology, and UKE now falls solely by gravity, thus without the help of TORI. TORI's mission is to support the fall of UKE and make sure that he will fall on his back or the side safely. In the tournament the control of UKE's is very important, because TORI has the option to slide with UKE together on the ground to use a control technique (Pinning, Choking, Arm locking) to force UKE to give finally.
TORI do not bend his upper body during the support of UKE's fall, otherwise his center of gravity will be shifted, and he may lose the balance - a safe cushion of UKE's fall would not be possible. Instead TORI goes, with a vertical upper body, lightly to his knees and builds at the right time a pull to the top up, which slows UKE's fall and ensure the control of UKE.