(*(void (*)())( (U8 code *)0x0000 ))(); // JMP 0x0000

0x0000 is casted to a U8 (maybe unsigned char) pointer
to code memory, then casted again to a pointer to a
function returning void. The most left * takes the
content, which is 0x0000 and the () most right is
the function call. This explanation might not be
perfect.

The example seems to be too complex, the same can
be done with

((void (code*) (void)) 0x0000)();

which I used in the past in two parts

#define RESET ((void (code*) (void)) 0x0000)

somewhere in a header file and then use it like

RESET();

which just looks nice in a C source code.

But now, something completly different. Make sure
this code it not used from inside an interrupt
function or nested interrupt function. Because this
is no real reset, from inside interrupts you must so
some RETI instructions (as many as the current nesting
level) to 'unlock' the interrupt logic.

Which is I prefer a real watchdog to do the reset.