You can mark an interval as "ACC" to indicate "acceleration". ACC allows you to add short "bursts" in the same gear, but with higher RPM to pick up the intensity and get your heart pumping.
Here is a video that will show you how to add "ACC"
You mark an interval as "ACC" by double-clicking the lowest part of the interval. The black stripes shown below will appear to indicate "ACC". Note that "ACC" will overrule any RPM input. You can remove "ACC" by double-clicking the interval again in the same spot.