The chopping part of this process went on for much longer than this video shows. You need to hack the mango a lot before you can slice it vertically to get the julienne pieces. The mango must be very hard and underripe, or it will be impossible to slice in this way.
I picked up this trick not from an auntie or friend, no: in Tranh Anh Hung’s film, the Scent of Green Papaya, the serving girl protagonist’s job is to slice up a green papaya with a machete for the family she works for. The Papayas grow in the family courtyard, and it’s a task we see her performing regularly as a young girl and an adult. When Lily from Longdan supermarket gave me this recipe, she told me she thought mango was better than papaya, being both cheaper and sweeter!
NB: Feats like these are for show-offs and cheap-skates. When in doubt, use a proper Julienne Shredder or a grater. Don’t chop off your fingers.