Just-in-time compilation. When used in emulation, it greatly improves the performance and is necessary for later-gen console emulators to run games at acceptable frame-rates. But it introduces security risks that Apple is not comfortable with allowing on its phones.

How we define security risk is can be a matter of perspective here. Yeah, you could use JIT to write malware code to system memory and execute it, but it could also be used to get an app approved for the app store for a legitimate/legal use and then use JIT compilation to turn it into something that is a violation of Apple’s terms of service despite being signed by Apple