What happens when Phineas and Ferb decide to do some Jackass? Correct. A roof top slide with looping. Only problem is that the gravity energy is not sufficient to get these guys to make the full loop. So after the first 25 secs we one of these geeks make a hard fall. The video in this posts starts right after this. (If you like these things just slide back to the beginning)
These guys solve the problem elegantly by adding some initial kinetic energy in the form of a truck pull. Now it works like a charm.
REMEMBER: DO NOT TRY THIS AT HOME! (Gravity WILL punch you in the face!)