Nexuses are bracelets invented by Lady Iskra that all Elvin children have to wear until their concentration is perfected and they can safely Light Leap (Usually around 14 or 15 years of age). Light Leaping is a method of transportation the elves use where they travel on a beam of light created by Leaping Crystals held up to the sun. If an elf is not wearing a nexus and their concentration breaks while light leaping, they may die by fading away. Fading happens when an elf loses too much of themselves in a leap, and they quite literally fade away into the light. There is a meter on the back of the nexus that measures how much concentration an elf has; when the meter is full the cuff can be unlocked, though in Neverseen, Alden uses a small disc to push their meters to full, and Dex says that he was tempted to do that himself. Biana Vacker is the youngest elf to get her nexus unlocked, younger than the previous elf, Fitz Vacker, her brother. Sophie used to wear two nexuses because Elwin was afraid of her fading away after when she almost faded one time. Nexuses are described as a cuff with a crystal set in the center. Like many Elvin things, they are typically decorated and intricate. Nexuses usually contain jewels and crystals.
The force field a Nexus creates when an elf leaps can be tracked, which is why they dispensed of them in Neverseen.