When the engine quits the model bleeds stored energy. Heading into the wind the drag coefficient of the model and the lines cause the model to bleed energy at a faster rate. When the model gets within one wing span distance it comes in to the ground effect region. This produces additional lift. So you combine the reduced air speed, higher energy bleed, and ground effect. These conspire in the loss of line tension, slower forward speed and higher lift. This results in those hard to control "Balloon" and bouncing landings unless you actively and consciously fly the model towards the ground. Whipping helps add energy back into the model which helps maintain line tension and some forward air speed.
Models that are less draggy whip better than those with higher drag, funny that heavier models also are better at this because they have more stored energy to shed than lighter models due to the need for more power to get them to move in the first place.
When the model is dead up wind and the breeze is blowing parallel (cross wind) to the wing the air speed that the wings see is cut, effectively the airspeed of the forward motion of the model minus the speed of the wind. As the model has not yet reached the down wind leg, where the wind helps add some energy back to the model (push it) and it is past the upwind leg where the model is bleeding more energy yet the effective airspeed the wings see is the forward speed of the model plus the wind speed, results in a null zone where things can get really tricky.
Its a balancing act, impart enough energy by whipping and help from the wind during the down wind leg to get have enough energy to bleed off on the upwind leg so you have enough line tension to get past that null zone.