Many large game and sports anglers often back down on a fish at the end of the fight. This means that the motor is gunned and reversed back towards the fish to bring it within gaffing or tagging range.
As the boat backs down onto the fish, the propellers send a tremendous pulse of water energy back from the boat towards the fish. With the sophisticated sensory receptors along the lateral line and the flanks of the fish, the fish is not only easily able to detect the presence of the fish, but is often panicked by the pressure waves.
As the pressure wave bounces off the flank of the fish, it often goes into high panic alert. All of its sensory organs scream in panic, that something big is coming towards it. In the underwater world, all creatures create pressure waves and anything big enough to create the pressure wave of a boat backing down onto even a large pelagic fish, must be capable of eating it. Accordingly, the fish has no option but to respond and try to escape.
This is often the reason why many large fish, suddenly find a new lease on life and power away from the approaching boat. It is constrained by the line, but constantly feels the approaching pressure waves, from the approaching boat. It has no other option, but to use all of its remaining energy to try and escape.
This is important for the gaffing of all species. It is particularly important for potential tag and release species, where to ensure the maximum chance of survival, the fish needs to be brought to the boat in good condition. By depleting all reserves of energy, the fish is not only well and truly beaten, but in danger of dying, even if released.
By bring the fish alongside the boat, there is minimal pressure wave or surface disruption, caused by the boat. This means that the fish can usually be brought much closer to the boat and in a much shorter time, than by attempting to take the fish from the stern. For tag and release species this can significantly increase the chances of survival. For all other species, it is simply a much easier and safer technique to adopt.