Chai Kick is Apachai Hopachai's ultimate technique and finisher. The technique seems to be a normal vertical kick upward that sends the opponent flying. However, when Apachai demonstrated with one of Akisame's rock statues Apachai said that the statue won't come down anymore because the rock had already blown apart into dust. Though usually vertical, sometimes the kick is done horizontally, as well.
Apachai supposedly named this technique, along with his Apa Punch, after Agaard Jum Sai taught him to put all his force into his punch. This was later used by Kenichi in his fight against Tirawit, in which he uses the Chai Kick as his finishing move. It can be speculated that Agaard knows a variation of this technique as well from teaching it to Apachai, though it has likely been absorbed into, and is hence indistinguishable from, his normal kicks after he joined the Satsujinken.