In many areas of the Pokémon games (except Sun and Moon), wild Pokémon of different species don't appear at the same level. For example, in Santalune Forest, Pansage, Pansear and Panpour only appear at level 4, but there are Pokémon in this forest that appear at lower levels (2 or 3).
The Repel Trick consists in using a Repel (or Super Repel, or Max Repel) when the first Pokémon of your party has a specific level (for example, 4 if you are hunting for the Unova's elemental monkeys at Santalune Forest ; don't forget to give a Smoke Ball to this Pokémon, so that it can flee without problems !). This level is always higher than the lowest level of the area, but lower than (or equal to) the highest level of the area.
As a result, you won't encounter some Pokémon that normally appear in the area. That means that you will encounter less Pokémon (but you can correct that if the Pokémon in the first place of your party has the ability Illuminate, or if you use a White Flute or an Encounter Power), but that the encounter rate of each Pokémon of the area will change.
For example, in Santalune Forest, the normal encounter rates are : 20% Weedle(X)/Caterpie(Y) ; 20% Scatterbug ; 10% Caterpie(X)/Weedle(Y) ; 10% Pansage ; 10% Pansear ; 10% Panpour ; 10% Fletchling ; 6% Pikachu ; 4% Kakuna(X)/Metapod(Y). But with the Repel Trick at level 4, the encounter rates (rounded to two digits after the decimal point) become : 22,22% Pansage ; 22,22% Pansear ; 22,22% Panpour ; 22,22% Fletchling ; 8,89% Kakuna(X)/Metapod(Y) ; 2,22% Pikachu.
To conclude, here is another example. In Pattern Bush (Pokémon Firered and Leafgreen), the normal encounter rates are : 30% Spinarak(FR)/Ledyba(LG) ; 20% Kakuna(FR)/Metapod(LG) ; 20% Heracross ; 10% Caterpie ; 10% Weedle ; 5% Ledyba(FR)/Spinarak(LG) ; 5% Metapod(FR)/Kakuna(LG). But with the Repel Trick at level 15, the encounter rates become... 100% Heracross !
Congratulations for your shiny Shinx !
I have already tried chaining with the Pokéradar in Pokémon Pearl, but my longest chain broke around 25 Voltorb (I can't check the exact number because it was on a save I have deleted). Anyway, now I usually do full-odds shiny hunting, so it doesn't matter.
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