He wants to be able to reboot the phone without having to hold down the power button , then slide down... wait for it to power off... hold down the power button to turn it back on..
Like on android you can press the power button and select "restart"
This is so missing in Windows 10 mobile.
to answer your question there is an app "device diagonistic hub" but i believe you need to pay for that feature of the app.
Just as ttsoldier said, I'm looking for that option just so I can instantly restart my phone without having to shut it down first via power button then turn it on again via power button. It's a simple but very useful option which I believe devs should look into.