There are four transport coils in the sub hopper.
Toner from the toner cartridge is sent to the large transport coils [A] that agitate toner. Between the large transport coils [A] and small transport coils [B], there is an open port [C] from which toner moves to the small transport coils.
The small transport coils [B] do not agitate toner but send it to the development unit through the supply port [D] for the development unit.
The large transport coils and the small transport coils differ in their blade pitch. The large transport coils [A] have a larger pitch to agitate toner easily. The small transport coils [B] have a smaller pitch so as not to supply too much toner at one time; this also means that even when the toner end sensor detects toner end, there is more toner held in the bottom of the sub hopper than if the coils were larger. This means that even when the toner cartridge becomes empty, toner auto-supply from the sub hopper continues, and you can replace a toner cartridge even during operation, which leads to reduced downtime.
400 sheets (A4/5% chart) can be printed after near-end.