E-writer conserves battery life and power consumption. It all depends on the structure of the LCD (Liquid Crystal Display). In resistive type E-writer, the display has layers of electrodes with a layer of liquid crystals sandwiched between them. The liquid crystal appearance changes depending on how they are aligned within the layers. Generally, liquid crystals are transparent. But when you apply pressure on the resistive type E-writer, the gap between the electrodes decreases and the liquid crystals flow into a new alignment, becoming reflective. When you put a dark background behind the screen and write some words by putting pressure on the touch-screen by using your finger or stylus, the liquid crystal alignment changes and it writes on the reflective layer.
In basic E-writers, you consume its battery power in two cases, either when you start its screen or you want to erase its screen. These two operations are rearranging the liquid crystal alignment.