Humminbird ProFlasher Operation Manual page 24

Broken but continuously displayed signals near the bottom return indicate vegetation or ground
cover.
It is often possible to see a "second return" from the
bottom. This is usually seen in shallower water when
the sensitivity is adjusted higher than normal. The
sonar signal is transmitted downward from the boat
and is reflected from the bottom back up. As the Pro
Flasher senses this returned signal, it is displayed on-
screen. The returned signal however is again reflected
off the surface of the water and then the bottom giving
a faint "second return" from the bottom. A second
return is always exactly twice the depth and much
weaker than that of the original return, so it is easy to spot on the dial.
Fish are easy to differentiate on the Pro Flasher because they are usually only displayed briefly.
Only a stationary fish directly under a stationary boat will continue to reflect the sonar signal.