What fundamental technique in jazz involves varying the rhythms of notes of a melody?

The fundamental technique in jazz that involves varying the rhythms of notes in a melody is syncopation. This technique provides a sense of surprise and excitement in music by placing emphasis on off-beats or weak beats, which creates an unpredictable and dynamic rhythmic structure. Syncopation is critical in jazz as it allows musicians to break free from strict adherence to a steady beat, encouraging creativity and individual expression within the performance.

This emphasis on rhythm can dramatically change the feeling of a melody, often making it feel more lively and engaging. It allows for a playful interaction between the rhythm of the melody and the underlying pulse, creating a distinctively jazzy feel. Syncopation is a foundational aspect of many jazz styles and is key to the genre's rich complexity and improvisational nature.

In contrast, the other techniques, while important in their own right, do not specifically focus on the variation of rhythms in a melody. Polyrhythm refers to the simultaneous use of contrasting rhythms, harmony deals with the combination of different musical notes played or sung simultaneously, and riffing typically involves repeating a short melodic or rhythmic phrase. These concepts, although related to jazz, do not capture the essence of varying rhythms in the way that syncopation does.