Okay, since I was super excited about the implications of this move,
and serebii happened to have incorrect information on it. I recruited
Chico the Parakeet to help me test the move Lash Out. (also thanks to
xJoelituh for getting me off my duff to actually test this.)
Rather than linking the 8 or so videos, I've included them in this
Drive link.
The details of the testing are below:
First test -
Turn 1:
Dugtrio and Toxapex lead versus Arcanine and Stuntank lead. No stat drops. Dugtrio sets Stealth Rocks. Stuntank does 34 to Toxapex with Lash Out.
Turn 2:
Arcanine drops Stuntank's speed with bulldoze. Stunktank does 72 damage to Toxapex.
This proves it procs off allies lowering your stats.
Dugtrio faints and Growlithe switches in at the end of the turn, inflicting intimidate.
Turn 3:
Squirtle switches in and takes stealth rock damage.
This proves Serebii is incorrect, the move does not clear hazards. Since that's defog's effect, I think we all assumed that. Lash out deals 22 damage - this is at -1, so that's 75 BP.
This proves that intimidate at the end of the previous turn does not boost the move. (This is important later)
Second Test -
Turn 1:
Arcanine and Stuntank lead versus Toxapex and Growlithe lead. Intimidate activates. Lash out deals 48 to Toxapex (once again, this is -1, so that's 150BP).
This proves that intimidate lead does boost the power on turn 1.
Third Test -
Turn 1:
Arcanine and Stuntank lead versus Toxapex and Dugtrio. Dugtrio is swapped to Growlithe, intimidate activates. Lash out deals 42 to Toxapex (so 150BP).
This proves Lash Out activates on an intimidate switch-in.
Fourth Test -
Turn 1:
Arcanine and Stuntank Lead versus Toxapex and Dugtrio. Dugtrio is switched to Growlithe, intimidate activates. Arcanine uses bulldoze, Stuntank's speed is lowered (so two stat drops). Lash out deals 46 damage to Toxapex.
This proves the move won't double twice. (also kind of assumed this, but doesn't hurt to confirm it!)
Fifth Test -
Turn 1:
Arcanine and Stuntank Lead versus Toxapex and Dugtrio. Arcanine uses bulldoze, Stuntank's speed is dropped. Lash out deals 67 damage to Toxapex (Doubled, but a very low roll this time).
Turn 2:
No stat drops. Stuntank deals 33 damage to Toxapex (75BP at +0).
This proves that the boost will not persist.
Many of these I kind of assumed - the primary goal behind this was to test if it procced off allies reducing your stats, and it does. I forsee implications in VGC, as a certain fuzzy wrestler can operate offensively under trick room and is weak to bulldoze, so it can get +2 and start launching +2 150BP STAB Lash Outs under trick room.