Pro Tools gives you many options in the Tempo Operations Window. They can be a simple and constant tempo change or a linear change. Linear will ramp the tempo faster or slower in a perfect slope in a selected area. There are more complex options as well to choose from. There is a parabolic option for instance that will let you set the curve of your ramp faster or slower in tempo among other options that can be quite helpful. I cover some of these different methods and what they do.