It splits the workload over the number of available processors. Most processors support ( at least ) 8 threads these days .. and even first generation Intel Core i3/i5 processors ( from 2010 ) support 4 threads.
K
Yep, the R registers are 32-bit while the X registers are 64-bit. However, i'd recommend using x8-x18 ( they are the 64-bit equivalent to r0-r10 ) instead of x0-x7 as these are generally used for function arguments ( by compilers ).