Assuming you’re talking about this patch, here’s the code it’s using:
I’ve highlighted the part that checks the option byte to see if HP Bars & warnings are toggled off. Fortunately, Circles and/or Tequila did a good job annotating what does what.
I’d recommend reading through this code to see what it’s doing exactly. If you apply this patch to a fresh ROM and go to this routine (it’s hooked at 0x276b4) in no$gba, you can trace through the code and make non-permanent changes to get a feel for how stuff works. There’s a guide on how to use no$gba here. There’s more info on asm and even some example projects guiding you step-by-step in that tutorial as well.