The Z9’s electronic shutter is not a fundamentally different kind of shutter than earlier electronic shutters: it still uses sensor readout, typically in a rolling fashion rather than exposing the whole frame at once.

What changed is speed. The Z9 uses a very fast stacked CMOS sensor, an evolution of back-illuminated sensor design, where separate layers can handle different tasks more efficiently. That greatly increases readout speed.

Why that matters: older electronic shutters were often too slow, which caused visible rolling-shutter distortion, banding under artificial light, and poor or unusable flash sync. Those limits are why mechanical shutters were retained in pro cameras.

With the Z9-generation stacked sensor and much stronger onboard processing, readout is fast enough that rolling-shutter artifacts are greatly reduced—low enough to compete with or even improve upon some mechanical focal-plane shutters in practice. The faster readout also enables useful flash sync with the electronic shutter.

So the key advances are:

• much faster sensor readout
• stacked CMOS sensor architecture
• more processing power in a battery-powered camera

Those improvements finally made an electronic-only professional camera practical.