Rail Pressure tuning?

[Cuda]

So what are the differences of how pressure boxes, Smarty, ect modify rail pressure? Since some stack together great, and some cause issues.. What are the differences between each box/programmers in the way that they modify the rail pressure? Do some ramp quicker.. or some just have constant high pressure.. I'm just interested to find out the dynamics of Smarty rail pressure adjustments vs a plug in rail and Map harness.