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Safe Mode temporarily disables any third-party apps that you may have installed on your device. If you have some problem with your device, and it stops when in Safe Mode, that points to a third-party app as the cause of the trouble. You have then to identify and uninstall it.
To boot into Safe Mode, press the Power button as if to power down the device. Then long-press the Power Off Phone? message, and you will get the option to enter Safe Mode. You can exit it by a Power Off and Restart of the device.
'Safe Mode' appears at bottom left of your screen as a reminder that you are using that mode.
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