PDT · Pushdown transduction
Reverse the input
Pushes every input symbol, then pops the stack printing as it goes — so the output is the input reversed. The transduction w ↦ wᴿ needs a stack; no one-way FST can compute it.
The author’s examples, run
ab → accept · baabb → accept · bbaaab → accept · baaabab → accept · babaa → accept · a