If you haven't read the changelogs Cydia for Mobile substrate, then you probably didn't know that saurik implemented in this plugin a system similar to a function already available in Palm OS. This function allows users to prevent the loading of tweak extensions when an iDevice boots and if you have a problem with any tweak, you can start your terminal without it entering safe mode, freezing, etc. Basically, when an iDevice boots, you have to hold down the volume key + (volume up), and the mobilesubstrate will automatically block the loading of extensions.
The feature is not new, it has been available in Cydia for some time and chpwn discovered it after using a device that has the Palm OS operating system installed. Mobiles substrate is available for free in Cydia, but it only works if you have installed tweaks that use it. This function is available on all iDevices that have iOS and mobile substrate installed.
