I have always used untethered jailbreak on my iPhone and it’s also recommended over a tethered jailbreak. The only difference between these two is that tethered jailbreak will be removed once the phone is restarted, but currently when you have upgraded your firmware version to iOS5.1.0 you will not find untethered jailbreak for your iPhone 4 or 4S. Hence then only option you have is to use tethered jailbreak or don’t jailbreak. After upgrading my iPhone to iOS5.1.0 I used tethered jailbreak for the first and believe me I had a very bad experience and there was only one reason behind it.
If you are out on a drive for a long day and you do not get a chance to charge your phone in between then your phone will get switched off after being low at battery but then problem is you cannot use your phone until and unless you have a laptop which has access to internet and a data cable for your iPhone. Once your jailbreak your iPhone there will be certain tools like SBSettings or many more which you keep on your jailbreak iPhone, now when a tethered jailbreak iPhone is switched off, it loses its jailbreak patch and thus any of the tools which were installed on your jailbreak iPhone will stop working which in case of tools like SBSettings will lead to restarting of springboard again and again.
Because those tools like SBSettings will restart the springboard again and again when they crash on your iPhone and thus you will not be able to do anything on your iPhone after every 5 to 6 seconds the springboard will restart itself, you will not be able to do anything (means anything) on your iPhone. Even, when you will try to switch off your iPhone (to get rid of that restarting loop), you won’t be able to do that also. Most of the other users will say you to press your HOME and POWER button simultaneously to force shut down the phone but actually by doing so you are not shutting down the phone rather you putting that phone into a recovery mode.
So, now the question comes ‘what is the solution?’
There are 2 types of solution, first one will help you switch ON your iPhone is such a way that you will be able to use it (comfortably) until it gets switched off again and second solution is to get rid of this problem totally.
When you have jailbreak your iPhone on tethered mode then you must have used a tool on your system like the one shown below. On that same jailbreak tool you will find an option to give a tethered boot to your iPhone, now this is the only correct way to restart your iPhone when it is jailbroken under tethered mode. Beand your iPhone will return back to normal, but then as mentioned earlier you will need a laptop with internet access and iPhone data cable if you want to switch ON your iPhone normally.
But if you are fed-up with this problem and you want to get rid of this jailbreak right away, then restart your phone and while it is stuck in restart loop, connect it to the iTunes and restore it (That’s what I did) and from then onwards you can wait for untethered jailbreak to come.
Actually if you have noticed then while jailbreaking the device the tool says that every time you want to restart it you will have to give tethered boot to this device. Please let us know if this does not help you to get out of the loop