EFCS starts the exposure electronically and closes it with mechanical shutter. This has the advantage that the exposure can be completed faster than using fully electronic (silent) silent shutter if the mechanical shutter is faster than the electronic reading of the data from the sensor. Downsides are that there can be shading with tilt/shift lenses and asymmetric bokeh in some cases especially with fast shutter speeds. Most vibration is eliminated since the closing of the shutter affects the sharpness less than the opening. Nikon limits EFCS shutter speeds to 1/2000s ot slower.
Fully electronic shutter has the advatage of silence and no shutter-induced vibration but because in most cameras it takes a while to read the data from the sensor, fast-moving subjects can be distorted and flickering lighting can cause banding in the image. Also, in most implementations, flash cannot be used.
In the A1 the reading of the sensor data is fast enough that electronic shutter can be used as default.
