This is a triggered ability, not an activated ability. It doesn’t allow you to tap Reckless Racer whenever you want; rather, you need some other way of tapping it, such as by attacking or crewing a Vehicle.

For the ability to trigger, Reckless Racer has to actually change from untapped to tapped. If an effect attempts to tap it, but it was already tapped at the time, this ability won’t trigger.