North has 19 points and so should open this hand 1 diamond.
South has no suit to bid, but they do have 7 points and diamond support, and so should respond 2 diamonds.
North is now in a bit of a fix. They know they have a diamond fit with partner, but that partner has only 6-9 points. They might only have a combined 25 points, a bit thin for game in a minor. They might consider bidding invitationally in diamonds, but it would be nice to know if no trumps would be a viable contract. North has every suit covered except spades, so they want a way of asking partner if they have some spade cover.
The important thing in this auction is that diamonds have already been agreed. What this means is that North cannot want to bid spades naturally.
Think about it. If south had 4 or 5 of a major then they would have bid that at the 1 level instead of supporting diamonds. On this auction then, it is VERY unlikely that North has opened a minor and recieved support from partner but then find that they have a major fit too. North would have had to have opened with something like 6 diamonds and 5 spades. If they had a hand this shape with 19 points they may well consider opening 2 clubs.
So once south has supported diamonds, North knows that they will not find a spade fit. They would therefor have no reason to bid spades naturally. So bidding spades here must be artificial.
With a short suit trial bid, bidding 2 spades with the North hand says ‘I have good points and good cover in most suits, but I have a short suit in spades. Have you got any good spade stops partner?’.