Constant Increment.
Will increase the speed with the selected amount for the selected number of loops or until the target speed is reached.

Gradually Decrease Increment.
Will increase the speed somewhat more than the selected amount for the first few loops but will then gradually decrease the speed increment.
The idea is that the closer you are to the target speed, the more difficult each speed increment will be.
This means that the last loops will increase the speed less than the selected amount and that this hopefully makes it easier for you to reach the target speed.