Hi fellow Samplitude user. Just like they said, it's a software implimentation of the pre/post switches on an analog console. In the case of an aux send for reverb;
If set to post (most common), as you pull down the fader, you are proportionally reducing the reverb send too.
If set to pre (less common), the reverb send stays constant as the track volume comes down with the fader, resulting in proportionally more reverb as you bring down the track fader. A good example is toward the end of "Four Sticks" by Led Zepplin. As Plant's vocal fades, it gets wetter (more reverb). It has the effect of moving down a hallway or a tunnel or cave. In the right place it's a good effect.