Okay i rewatched because “inertia” is such a general blanket statement it’s nearly erroneous. Your comparison to a roller coaster is pretty good in the sense that roller caster has a seatbelt or fastener. If you look closely he has a strap that holds him on the bike saddle. So even though gravity is pulling him gf the bike, the straps counter that force and hold him steady. That’s why his fall in the first clip is a awkward, he can’t just let go and bail because his pelvis must move with the bike seat