Cell Substrate is a utility that enables Cydia interior construction and the real execution of quite a lot of tweaks and in addition helps run a "protected mode" in case some tweaks and extensions have bugged.
You can now disable Mobile Substrate entirely by holding down Volume Up when the mobile starts. This feature allows you to bypass the broken extensions (those which prevent the unit from starting) you have installed, so you can open Cydia and uninstall.
For devices that do not have a Volume Up button (the iPod touch first generation), you can also hold down the "Click" (not Volume Up) on your headset or headphones.
Indeed it is possible that you encounter problems with some tweaks and therefore you can not restart your device, then forced to restore. Now it's over, you run more risk, if your device will not boot because of a tweak you can just keep the volume up button (or when the hands free kit, if your volume buttons do not work either for iPod touch 1G, we must be supported in the middle) to disable the Substrate, the dependence of tweaks, which means that all your tweaks installed will no longer be active and your device will restart normally . Then you can access Cydia to remove the tweak failed. So you escape to the restoration.
Or simply to avoid this blunder, be careful what you install, read the description of the tweak.
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