The peduncle maneuver of a Humpback Whale is best described as a sideways thrust of the lower part of the whale’s body.
It is definitely one of the more powerful expressions of the Humpback whale.
My observations have been that the whale will most commonly perform this maneuver while another whale is close by so it could be interpreted as an aggressive maneuver however there have been times when I have seen the maneuver when it seems the whale is alone.
Female Humpbacks and calves also perform the maneuver so this is probably the mother teaching the calf the ways of a Humpback whale as both have been seen doing peduncles together.
The peduncle is usually a quick, powerful sideways thrust of the tail and lower body resulting in an amazing splash of a large volume of water.
Sometimes the peduncle can be more upwards than sideways which is the more powerful of the two descriptions, the tail and lower half of the body arcs out of the water very quickly with a huge spray of water usually obscuring the view of the tail for a moment. This is similiar to a tail slap but with far more force.
The lower half of the whale’s body is visible and usually the tail will smack back down into the water with great force.
In my opinion a Humpback Whale performs this maneuver both for aggression and also for play.