There used to be paperwork that one would hand over at the customs desk, but nothing really happened if you forgot. If it is all e-filings now, I guess it is literally just showing the passport. It is probably still pretty much without consequences if you don't pass by the counter. The counters in Haneda and Narita are in the immigration hall, by the back wall facing the immigration booths. Takes about 10 seconds to complete.

I don't know anyone who has been asked to show the purchases. In that aspect the Germans are more thorough, I don't think I have ever claimed tax free in Germany without showing that I had the items.