It started with somebody else's game. In a 2016 interview, Yuji Naka, the programmer behind Sonic the Hedgehog (1991), told how he saw Capcom's Ghouls 'n Ghosts at the 1988 Amusement Machine Show and couldn't work out how its hero walked up and down the hills. Back at the office he asked his boss to negotiate with Capcom so he could port it to the Mega Drive, to read the code. That hero could move 2 dots sideways every sixtieth of a second; he tried about 16, and it was incredibly fast. Then he made the slopes steeper and steeper, and noticed: if he joined them up, wouldn't it go all the way round? It did.

The trick in our own simplified version: give the runner one speed along the ground and let the ground's angle steer it. Up a slope, part of that speed goes up; up a wall, all of it; past the top, it runs upside down. Too slow, and at the top the ground would have to pull you in, so you drop. Naka said in 1997 that beginners couldn't do the loops and couldn't get their speed back, which is where Sonic 2's spin dash came from.

"What we had first, was smooth, rolling terrain and a character who could do 360 loops in that terrain." The hedgehog came after.

The track, the ball and the motion in this film are our own simplified version, built in code.

Which game's hidden trick should we cut open next?

Sources: Yuji Naka interviewed by Masayuki Kibe, Denfaminicogamer, 7 Jul 2016 (Japanese; our translation). Naka and Naoto Oshima, Sega Magazine, January 1997, and 1991 interviews, translated by shmuplations.com.
Unofficial. Not affiliated with or endorsed by Sega or Capcom.
Sounds and music: our own, made in code.