"force controlled" and "displacement controlled" loadcases

Hello all,

I have a question about the equations behind the difference between loadcases that use forces vs loadcases that use enforced displacements.
I am aware that these two loadcases lead to different results on Optistruct, but have found very few info in the help about it.

I did a simple test watching the behaviour of a part under torsion and bending when I try to optimize it on Optistruct (minimize mass, Von Mises static constraints, gauge optimization).
-When 'enforced displacements' are used, we obtain a lighter part ( decrease of the thickness ) but the stresses decrease as well ! This is totally illogical since a decrease of the thickness should necessarily lead to an