Kintaro's clones.
The Clone Technique, is a move where Kintaro creates many clones of himself that leap into the air and each take swings in the air in the hope of returning a shot that extremely difficult to see. First shown when he has his One-Point-Match against Echizen, and goes into Muga no Kyouchi to use Tachibana Kippei's Abare Dama which is a technique that creates multiple balls, the technique allowed Kintaro to create enough clones to swing at each ball that appeared until a clone eventually hit the real ball.
The technique also allows Kintaro to swing the racket in many places at once until he finds and returns a a shot that is difficult to see. For example in his Foreign Expedition Group Challenge against Hakamada Izou, he uses this technique to counter and defeat Hakamada Izou's Vanish technique which lifts dust off the surface and makes the ball nearly impossible to see.
Echizen Ryoga uses what is portrayed as the exact same move against Akutsu Jin when the two briefly rally somewhat aggressively with 10 balls outside the U-17 Camp toilets.