I have a 798c SI, my screen is smaller but the units are similar.
Side imaging is the same as down imaging except for the shape of the beam and the way it is displayed. Nothing Magic involved.
Two side pulses are are sent, one to each side in an extremely narrow beam.
You must be at trolling/idle speeds. 2-6mph. If stopped/too slow the display is nearly meaningless.
The display prints, based on when the pulse returned.
A fish 100 feet straight down and another 100 feet to the side will echo at the same time and display as a swimming pair.
Side image range is about 240 feet in radius. In 250 feet of water, there will be no bottom return.
In 150 feet of water you will be able to see about 200 feet of bottom detail to either side. 400 feet side to side.
A sunken boat actually looks like an identifiable boat. Really cool!
Big fish actually look like a big fish, not arched returns.
Overall, Side imaging operates as advertised, it will take a little time to learn to interpret the display.
It is a bit distorted from what you may be expecting.
A vertical pole out to the side will appear to be leaning toward the boat because the top of the pole is closer than the bottom.