From Tekkenpedia

Twin Pistons
Command	d/f+1+2 WS+1,2 (T3 onwards for Kazuya and Jin)
Stance
Damage	10,15 (T3, TTT and T5) 12,15 (Kazuya TTT-T4) 8,21 (Heihachi T3-TTT) 8,17 (Heihachi T4-T5) 8,15 (Heihachi T5:DR, Devil TTT)
Hit Range	mm
Properties	JG
Move Frames		Hit Advantage
{{{Frames}}}		{{{Hit}}}
Block Advantage		CH Advantage
{{{Block}}}		{{{CH}}}

Twin Pistons 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.

Ever since Tekken 2, Heihachi's Twin Pistons have always looked different from Kazuya's and Jin's. In Tekken 6, he finally lost it completely.

[edit] 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 moved though, which is why it should be used after a low parry or a ducked high attack.

[edit] 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.