For those new to bitshifting, here are the results of the "stuff" operation that @dahfizz describes:

  'a' - 'a' = 0, so take 1 and shift it left 0 times:
  00000000000000000000000000000001 = 'a'

  'b' - 'a' = 1, so take 1 and shift it left 1 time:
  00000000000000000000000000000010 = 'b'

  'c' - 'a' = 2, so take 1 and shift it left 2 times:
  00000000000000000000000000000100 = 'c'

(I'm not sure why the post's author chose to use 10000000000000000000000000000000 as their example for 'a' rather than the above which IIUC is how the code actually works.)