Hard will COUPLE something to the surface below, soft will DECOUPLE it from the surface.
In the first case, for example with spikes, you would increase the pressure per surface area, which effectively makes the surface part of the speaker (or any component). Coupling works best on real wooden floors, where the vibrations from the speakers can be transmitted and damped. It also works reasonably well on concrete, but it could show up resonances in the speakers, as the energy has nowhere to go.
Decoupling means you isolate the vibrations from the floor to the component and vice versa. More or less all or most of the vibrations will be absorbed in the damping material and transformed into heat.
For MartinLogan speakers, I find the best way to have them sing their best is to use spikes, or better yet, ETC spikes.