What is actually happening when you hard brick a phone? I've never hard bricked a phone. I've never seen one either. But hard bricking is everyone's worst nightmare.
I wan't to know what exactly happens when you hard brick your phone. What happens that makes it irrecoverable without professional help?
To find this out, I needed to know how flashing of ROMs work. For this, I downloaded the data sheet for my phone's chipset (an MTK6589, whose datasheet can be found here) and then found out that:

The USB data lines D+ and D- and VCC goes directly to the application processer.
There is an external PMIC for application processor that handles the power ON and OFF of the handset. On this IC there are two pins(KP_KOL0 and KP_ROM0) which when are 0, triggers USB download without battery.
The memory (eMMC or NAND Flash) to which the files are downloaded is also connected to the Processor via a External Memory Interface (EMI).

So this is what (I think) happens when you flash a wrong ROM:
Though I don't know what results in 0V on pins KP_KOL0 and KP_ROM0, but this happens when you want to download a new ROM, and the application processor switches/starts in Download mode.
I've googled what happens when you hard brick a phone. All I get are noob posts telling how or how not to hard brick your phone. Most of the posts say that when you flash the wrong ROM, the phone may get hard bricked. My question is,
What happens when you flash a wrong ROM that makes the phone hard bricked?
Or why does the processor become not responding at all when you hard brick it?
It has nothing to do with the processor, unless you've fried it.

Usually, the only software-based hard brick that is possible is when you overwrite the bootloader.  Without a (working) bootloader, the phone (a) cannot get to the point where it would load the operating system and/or (b) cannot have working software loaded onto it.

If you get into this state you need to access the hardware more directly to write a new bootloader, often using JTAG.