Similar to a simple pressure boundary condition with the follower pressure boundary condition type a distributed load normal to the applied surfaces elements is applied. In contrast to the pressure boundary condition the follower pressure takes care of the current deformed state of the surface and the change in direction of the normals as well as the area change is taken into account. This dependence on the deformed geometry makes this a non-linear boundary condition type and it is thus not applicable in a liner analysis (like a linear static or harmonic analysis).
Comparison between pressure and follower pressure boundary conditions in a nonlinear study:
Top: Follower pressure applied to the top face of the beam. Bottom: Standard pressure applied to the top face of the beam.