DOWNLOAD MECHANICS YAP:
If you play or watch BW, you might have noticed that Genesect gets either an Atk or spA boost on entry, via its ability
Download. This boost and its mechanics are critically important to playing BW, but they aren't immediately obvious.
Entry Text:
On entry, Download and other entry-text abilities (relevant ones being Intimidate, Teravolt, Sand Stream) have the same priority, so their order is based on speed. This means, in particular, that Genesect can scout for opposing speed tiers--if you're running standard CBG Genesect then you'll instantly know if opposing LandoT, Kyube, and Ttar are scarf or not.
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(Ttar's Sand Stream procs before Download, meaning it's scarf AND I should be wary of being sniped by a fire move)
Boost Mechanics:
CBG typically wants an Atk boost, functionally equivalent to its item. In doubles, Downlaod works by adding the two opponents' Def and Spdef, and taking the boost corresponding to the lower sum. In a tie, Download takes a spA boost.
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(CBG and LandoT EV spreads, side-by-side. A small EVs tweak results in opposing Genesect coming in with -1 Atk, from Intimidate)
This is especially relevant for teambuilding--it makes very high-SpDef mons like Jellicent and Latios almost-automatic +1 Atk boosts, and hence making Genesect more threatening. On the other hand, high-Def mons like Metagross and Conkeldurr lean Genesect towards a +1 spA boost--one reason that flamethrower is more common than blaze kick, even on CBG Genesect. You can also closely examine your EV spreads to see if small tweaks will affect Genesect's Download boosts vs particular pairs.