As he says "My ultimate technique...", Tien charges a great deal of energy in his right hand. Then, he brings his hand forward and fires the energy in the form of a Tri-Beam against the opponent, inflicting a great deal of damage.
Tien uses this attack during the battle against Nappa. Enraged that Chiaotzu's suicidal attack on Nappa was in vain, Tien uses all of his remaining energy and fires a Spirit Tri-Beam up at Nappa in a final attempt to kill him. Though the Spirit Tri-Beam manages to do a number on Nappa's armor, it fails to inflict any real damage (though Vegeta notes that, had Nappa not blocked the attack, it might have finished him off) and Tien, having used up his remaining energy, died.
Appearances in games
Spirit Tri-Beam was named in Dragon Ball: Raging Blast, where it appears as one of Tien's Super Attacks that is unlocked by wishing it from Shenron. It also appears in Dragon Ball: Raging Blast 2, Dragon Ball Z: Ultimate Tenkaichi, Dragon Ball Z Battle of Z, and Dragon Ball Z: Shin Butōden.
In Dragon Ball Xenoverse, performing the Tri-Beam on the ground causes the user to fire it with one hand, turning it into a Spirit Tri-Beam. Thus the Future Warrior and Cell Jr. can use the Spirit Tri-Beam as well.
In Dragon Ball Xenoverse 2, like in Xenoverse, performing the Tri-Beam on the ground will cause the user to perform the Spirit Tri-Beam and thus can be used by the Future Warrior (Xenoverse 2) as well (in addition to Tien and Cell Jr.).