Nen Circuit is made up of three separate Hatsu that work together. It’s made for a Manipulator/Emitter nen user.

Nen Charger - The user emits a small field of nen around their body like ken or ryu. It passively absorbs a small percentage of all nen that passes through it, and stores it in Nen Batteries. If the user is hit with an attack, it will absorb 20% of the attack’s nen. If the user is attacking, it will absorb 10% of the attack’s nen. If the user is not attacking or being attacked, it absorbs the user’s nen at the same rate as their body produces it. Nen Charger stops absorbing nen if they activate zetsu.

Nen Batteries - Nen constructs that store nen for later use, and remain concealed until used. They can slowly recharge a person’s expended nen, or can be instantly emptied to empower an ability. However, the aura type stored in the battery must match the recipient’s aura type, or else the amount of nen they receive will diminish. The further the recipient’s aura type is from the type in the battery, the less efficient the charge.

Nen Railguns - The user creates either three small nen railguns, or one large nen railgun. These railguns can be carried or set up as stationary turrets. They fire nen projectiles, and will keep their sights trained on one target the user can see. The turrets can track a target that the user sees, even if they do not have line of sight. Nen Turrets are powered by Nen Batteries; the more batteries they’re given, the greater the speed, range, and damage of their projectiles.

TL;DR:

Nen Charger - Absorb nen & store it in Nen Batteries.

Nen Batteries - Stores absorbed nen for later use. Can be given to self, others, or Nen Railguns.

Nen Railguns - Guns that fire nen projectiles at a target the user can see.