Twin Pistons

Twin Pistons
Command	d/f+1,2 WS+1,2 (TTT-present)
Stance	While rising or in crouch position
Damage	10,15 (TK) 10,15 (Kazuya, Devil TK2) 10,28 (Heihachi TK2) 10,15 (TK3, TTT and TK5) 12,15 (Kazuya TTT-TK4) 8,21 (Heihachi TK3-TTT) 8,17 (Heihachi TK4-5) 8,15 (Heihachi T5:DR, Devil TTT)
Hit Range	mm
Properties	JG
Move Frames		Hit Advantage
11 (TTT)		+9 KD (TTT)
Block Advantage		CH Advantage
-2 -6 (TTT)		+9 KD (TTT)

Twin Pistons, called Double Upper (ダブルアッパー daburu appā) in the Japanese version, is an attack used in Mishima Style Fighting Karate and can be used by all its practicioners, with the exception of Jinpachi. The move has existed since the first game in the Tekken series. Ogre and True Ogre have a similar looking attack with the same command called Double Elbow, however, they use their elbows (hence the name) while Twin Pistons utilizes the fists. Also, Double Elbow does not launch the opponent.

The original input for Kazuya was df+1,2; ever since Tekken Tag Tournament, it's instead being WS 1,2. In Tekken Tag Tournament 2 when Kazuya transformed into his devil form which retain his old df+1,2 command attack move, it is dubbed as Devil Pistons and cannot use any regular Kazuya’s df+1 followup attacks aside this Pistons move while in this form.

Ever since Tekken 2, Heihachi's Twin Pistons have always looked different from Kazuya's and Jin's; the second hit in the form of his Demon Uppercut. As of Tekken 6, he no longer have this move, while only be available during Rage Art move only.

Several characters such as Kuma, Miguel and Bob have right-left variations of this move which are slower and are much more unsafe.

Claudio also has the similar-looking move to Twin Pistons, but it's slower at 15 frames and also launch punishable.

When to use it

"Back in the day", when Twin Pistons was a simple d/f+1,2 move, it was a natural, guaranteed combo, meaning that if d/f+1 hits, 2 would be guaranteed for a juggle. Its speed, follow-up, and ease of use made the first half of Twin Pistons a highly valued poke, which in Heihachi's case, continued up until Tekken 6 where it was removed. So in the case of a d/f+1,2 Twin Piston, it can essentially be used whenever. Use d/f+1 as a poke, if it misses, stop, if it connects, go for a juggle.

When Twin Pistons became a While Rising move, it also became less frequent because it could not just be "whipped out" as a poke (does not apply to Heihachi of course). It was still prized as an excellent While Rising move though, which is why it should be used after a low parry or a ducked high attack.

From Tekken 3 and onwards, Twin Pistons can be executed from a wavedash as a mixup with the Hell Sweep, which is a low. So if the opponent expects the Hell Sweep, they will get hit by the Twin Pistons. Many skilled Mishima players (most notably Kazuya players) use this mixup with notable success, though it is considered very hard to pull off consistently. In Tekken 6, this mixup has been toned down, as Twin Pistons does not track to the left anymore, the mixup can easily be avoided by sidestepping left.

Trivia

• It's interesting to note, that in Tekken Tag Tournament, despite being the same entity, the damage and commands for Kazuya's and Devil's Twin Pistons are different.
• Claudio's aforementioned launcher and the characters with right-left 2-hit launchers (see above) are referred to as Twin Pistons despite not having the name, because of the similar role.
• Even though Heihachi lost this move in Tekken 6, in Tekken 7 he uses this move in the rage art as second strike to kick the opponent in the air.