# What drives N1 during a jet engine start?

A typical jet engine start sequence that I understand is as follows:

ENG Start Switch to Ground → Start Valve open → N2 increases → N1 increases → ENG Start lever to Idle (when N2 has reached a certain number) → Fuel flow starts → EGT rises → ENG Start Switch cutout

As far as I understand, N1 and N2 are mechanically seprated. If that's the case, what drives N1 before it starts to rotate from the following combustion?