Ok, so you would think that understanding the way Motion works would allow you to drop a retiming behavior onto the particle emitter. Unfortunately you can't and if you could then I wouldn't be writing this tutorial!

This is where the clever bit happens, just clone the layer by either hitting the K key or selecting it by navigating to almost the bottom of the object menu. If you haven't used the clone layer command before then this allows you to duplicate a layer and have certain parameters change on the clone when you change them on the original.