Motion sensing technologies enable musical interfaces where a performer moves their body "in the air" without manipulating or contacting a physical object. These interfaces work well when the movement and sound are smooth and continuous, but it has proven difficult to design a system which triggers discrete sounds with precision that allows for complex rhythmic performance. We conducted a study where participants perform ``air-drumming" gestures in time to rhythmic sounds. These movements are recorded, and the timing of various movement features with respect to the onset of audio events is analyzed. A novel algorithm for detecting sudden changes in direction is used to find the end of the strike gesture. We find that these occur on average after the audio onset and that this timing varies with the tempo of the movement. Sharp peaks in magnitude acceleration occur before the audio onset and do not vary with tempo. These results suggest that detecting peaks in acceleration will lead to more naturally responsive air gesture instruments.