When DIO stole Jonathan's body, his left side was weaker due to Jonathan's body not being a vampire. When we see that Jotaro punches DIO in the shin, DIO uses his left leg. His left side wasn't healed yet, and when he kicks him, Jotaro sees the opening and pulls all his power into his fist. See, the more Jotaro gets mad, the more calculated he is. In part 1, DIO uses anger to successfully make Jonathan mad enough to punch him in the boxing match, and to be able to dodge his attacks when Jonathan dashes into the building, shouting "DIO...!" So that's the full demonstration of that.