MobileSubstrate Has Built-In Palm OS-Inspired Safety Feature
MobileSubstrate has a built-in Palm OS-inspired safety feature that can be used to prevent extensions from loading on boot, reports chpwn.
MobileSubstrate is the de facto framework created by saurik that allows 3rd-party developers to provide run-time patches ('MobileSubstrate extensions') to system functions.
Jailbreak tip: MobileSubstrate has a Palm OS-inspired safety feature -- hold volume up while rebooting and it won't load any extensions.
The feature is useful for determining if an extension is causing a problem with your iOS device and will let you bypass the loading of a faulty extension that prevents your device from booting.
Chpwn notes that he got the idea for the button combination from this Palm OS feature.