After creating a small Rasengan, Boruto subconsciously adds Lightning Release to it, before throwing it. As it glides, the nature transformation abruptly kicks in, resulting in the technique disappearing from sight. This adds an element of surprise to the attack, as an opponent may lower their guard, assuming that the technique has dispersed, only to find themselves struck with by the invisible attack.
The size of Boruto's Vanishing Rasengan has seen a significant increase after the Mujina Bandits Arc, where it was shown to be around the size of a regular Rasengan. With enough chakra added to it, the Rasengan can become giant.