MobileSubstrate is a Free jailbreak tweak in Cydia developed by Saurik. He is the person responsible for starting Cydia, Winterboard, and many other tweaks. He has developed many great tweaks in Cydia. MobileSubstrate is like a “safe mode” for the iOS device. Should the device crash it will restart in safe mobe through MobileSubstrate. Now Saurik has added a few awesome features to the tweak. My favorite is the ability to enter safe mode during reboot by holding down the volume up button.
If you haven’t used MobileSubstrate I would suggest you get familiar with it, if you are going to run a jailbroken device and try out new tweaks often. It will save you a lot of time and effort trying to figure out what is causing your device to crash. This is a great tool for jailbreakers of all types. Watch the video review for more information on MobileSubstrate.