What it does
Strength Sap is a Grass-type status move with two simultaneous effects: the target's Attack drops one stage, and the user heals HP equal to the target's current Attack stat — calculated after any stat changes already in effect, but before this move's own drop applies.
The combination is what makes it worth a moveset slot. You reduce incoming physical damage and replenish your HP in the same action. One move does what would normally take two separate support decisions.
The only failure condition is if the target is already at -6 Attack. Every other situation that might block a stat drop — Clear Body, Mist, and similar — still allows the healing to go through. The HP recovery is not conditional on the stat drop landing. That's worth remembering: even if the Attack drop gets blocked, you get the heal.
One interaction to check before using it: Contrary reverses stat changes, so a Pokémon with that ability would gain an Attack stage rather than lose one. The heal would still trigger, but you'd be buffing their physical damage in the process.
Grass-type status means this fails against Grass-type targets (type immunity) and triggers Sap Sipper to raise the opponent's Attack — the opposite of what you want.
Who runs it
Sinistcha (16%) is the primary user. As a Ghost/Grass type, it already shrugs off Normal and Fighting physical moves, and Strength Sap trims the remaining physical threat. Between its natural bulk and repeated heals, Sinistcha is extremely difficult to wear down. Its role in doubles is disruption and longevity — Strength Sap is the mechanism that keeps it in the game long enough to do that job.
Polteageist (9%) runs a similar concept. It leans more toward special attacking than Sinistcha, but Strength Sap still serves the same purpose: cutting physical damage and buying extra turns by healing back the chip it takes while attacking.
How to use it
The optimal target is a physical attacker whose Attack is currently high — the heal scales with their Attack stat, so a boosted or Choice Band physical attacker hit by this heals you for a lot while also cutting their damage significantly.
A typical doubles sequence: opponent's physical attacker comes in, Sinistcha uses Strength Sap, the Attack drops and Sinistcha jumps back up in HP. If the opponent wants to keep pressuring through that Pokémon, they have to account for the reduced damage output. If they switch, the Attack drop resets — but switching is still a concession, and Sinistcha keeps the health it recovered.
Pairing with an Intimidate user in doubles is particularly efficient: Intimidate drops the opposing Attack by one on switch-in, Strength Sap drops it another stage, and the heal hits while the target's Attack was still relatively high. That two-stage Attack drop combined with HP recovery in a single lead turn is difficult for physical attackers to deal with.
Tips & strategy
Don't sleep on the heal even against low-Attack targets. As long as the target has any Attack stat (which everything does), you recover some HP. The amount might be small if you're hitting a special attacker's physical Attack stat, but against the physical threats in the current meta, the heal is consistently meaningful.
Polteageist's special attack investment doesn't reduce the heal from Strength Sap — healing scales off the target's Attack, not your own stats. Both effects work independently of your build, so an offensively invested Polteageist gets full recovery value out of this move the same way a defensive set would.