I remember seeing a documentary on TV where two of Fats Waller's songs were compared, one from around 1930 and the other about five years later. In the first, the melody was very much on the beat; in the second, the melody lagged the main beat by just a shade. The development was described as being the introduction of swing. It gave a laid-back feel without the full syncopation of ragtime.

The alternative, where the melody leads the main beat by just a shade was demonstrated in the driving Latin music of Cuba which is almost impossible not to dance to.

DC