Introduction
Hold your horses, ladies and gentlemen, because this ain't what you're thinking it is! It's something much better! ...Kind of. The screen crunch is still pretty bad.
Sonic the Hedgehog GBA is a port by Simon "Stealth" Alexander Thomley of the original Sonic the Hedgehog to the Game Boy Advance. The proof of concept was created in early 2007 as a direct counter to Sonic the Hedgehog Genesis, SEGA's official "port" of Sonic 1 to the Game Boy Advance, and was publicly released for the 2008 Sonic Hacking Contest. It even won the 11000101 Trophy for the best technical work in a submission.
This port has added the spindash into Sonic 1, and you can also play as Tails or Knuckles by holding A+Start or B+Start at the title screen respectively. There is also a partially complete Labyrinth Zone that you can go into by holding L+Start at the title screen, and there are also two accessible Special Stages.
Explanations
Since this is a ground-up port of Sonic 1 by Stealth, its physics are pretty similar to later RSDK games such as the 2013 Sonic 1 & 2 mobile ports. In fact, it could actually be built on the RSDK engine due to way the camera works when moving at high speeds, and the presence of a perfect spindash, but I'm not sure. As such, the tricks used in this TAS are pretty similar to other speedruns and TASes of RSDK games.
Within the RSDK engine, there is a perfect spindash, which is a 6-frame spindash. However, it doesn't work in the way that you think it does. It's a lot faster than what you've seen in other speedruns or TASes. It seems to be around three times faster. I use these spindashes to go through the acts at high speeds. Aside from that though, there isn't much different about this port compared to the RSDK version of Sonic 1.