It's a translation.
A reflection across a line is like flipping that object like a pancake over the line. It gets turned upside-down.
When you flip the pancake again, now across a parallel line, it gets turned right-side up again, just in a new location.
8 mi/s = 12.8 km/s
At this rate, in 2 seconds Jupiter will travel 25.6 km.
