In your early runs, receiving a hover board will prompt you to use it immediately. You can activate it, though at any time by tapping twice quickly on the screen. Hover boards actually serve as a second line of defense against crashes and instead of resulting in a dire end for your run, the board will simply disappear if you crash and you get to continue off on your adventure.
One great thing about hover boards is that you can use it several times in a single run. There is a cooldown period within which you cannot use another board after the current one disappears. If you are far enough into your current run, this can become dangerously risky especially since the pace of the game will be faster and avoiding obstacles will be much more challenging.
Like keys that you can consume to save your runner and continue on with your run, utilizing the hover board buff and all other boosts should be saved for important runs. While you can freely use it on regular runs or practice runs, saving all you have for your leadership board entry is the wiser option.
Even on runs that you are especially preparing for, using hover boards indiscriminately is not the best utilization. Given that the boards can save you from crashes, the best time to use them is when you are certain that you will crash and be unable to avoid the obstacles coming your way. More importantly, if your current run is not even enough to beat your current record, then you might as well go a bit farther before expending the resources you worked hard on saving.