What are the differences between hand and hold luggage screening?
As ratchet freak pointed out you do have some more flexibility in ensuring that a small cellphone-sized explosive device is positioned to cause maximum damage if it's in your carry-on baggage as opposed to if it's in checked baggage/cargo. As such there is an inherently higher risk here.  

Counterbalancing that risk is the fact that we already have good screening tools to detect explosives or other anomalous things (like say knives or tear gas) concealed in what purports to be an electronic device:


Electronic devices are subject to X-Ray inspection.
Trained operators can spot quite a few anomalies using these "classic" tools (though rechargeable batteries can often look like explosives).
Explosive Trace Detection machines are already a part of security screening.
There are also swab machines which operate on a similar principle. They are not foolproof (they have false positive results occasionally, and may miss well-packed explosives), but they're a good tool.
Airport security has already asks to see suspect devices power on.
I had an old Compaq laptop with a completely useless battery, and security would let me plug it in if the battery was dead.


These same techniques (and others) are available for use on checked baggage.
Checked baggage may also be opened for inspection, which could conceivably include powering on any electronic devices in checked baggage (but note that the TSA recommends you not pack your electronics in checked baggage).



It's worth pointing out that, at least from a technical standpoint, faking a "power-on" is trivial with today's technology. Unless they're making folks do a full functional demonstration of the device in question a motivated individual with malicious intent would have no problem making a device that's mostly filled with explosives but can still "power on" and display a home screen, which makes the additional security benefit here dubious, at least in my estimation.