You clear the display by calling display.clearScreen().
virtual void clearScreen(uint8_t value = 0xFF) = 0; // init controller memory and screen (default white)
virtual void writeScreenBuffer(uint8_t value = 0xFF) = 0; // init controller memory (default white)
You turn of the booster circuit for panel driving voltages by calling display.powerOff().
You can hibernate the controller for lowest power use by calling display.hibernate().
virtual void powerOff() = 0; // turns off generation of panel driving voltages, avoids screen fading over time
virtual void hibernate() = 0; // turns powerOff() and sets controller to deep sleep for minimum power use, ONLY if wakeable by RST (rst >= 0)
The easiest way to find all methods is by looking at GxEPD2_GFX.h
With the Waveshare board you can pull RST low to turn off power completely by the "clever" reset circuit. I don't recommend this.
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Jean-Marc