Beats have front and back sides

From childhood we are encouraged to tap the beat with our feet and play on the beat. Then, as we mature musically, we find out that playing on the front or the back of the beat can sometimes make the best and most interesting and engaging music. Timing in Vivaldi and Wieniawski is different; what is right in one is flat-out wrong in the other. Sometimes it is appropriate to play exactly on the beat. Other times, it is appropriate to stretch the beat, delay the beat, or push the beat. Making these choices requires forethought. And it is fun.