A creation expression executes once for each particle whose age is 0 when Maya evaluates dynamics. Maya evaluates dynamics whenever the animation time or the particle’s current time changes, and it’s greater than or equal to the particle object’s Start Frame setting—frame 1 by default.

The animation time changes when you rewind, play, or otherwise change the current frame displayed.

An emitted particle’s age is 0 in the frame where it’s emitted.

Particles created with the Particle Tool have an age of 0 on and before the Start Frame. With the default animation frame range and Start Frame, rewinding an animation to frame 1 returns such particles to age 0.

If you set the Time Slider’s start frame higher than the Start Frame, be aware that rewinding the animation might cause the age of particles to be greater than 0. If this occurs, the creation rule for the particles won’t execute.