Thanks for explainig! I've already figured out that my mistake was to think that each new letter sticks to the end of string

Hey, take a closer look at string(r) + word part. Here we are "attaching" letter by letter BEFORE the existing word.

Example for "HELLO":

1. "H" + ""
2. "E" + "H"
3. "L" + "EH"
4. "L" + "LEH"
5. "O" + "LLEH"

I hope it makes sense:

I really can't figure out where words were reversed?