there are a few different ways of controlling the explosion — or force, as the set of parameters is called — in shatter.
for demo purposes, add a 3d camera to the comp where you want to create your shatter and then change the shatter’s Camera System setting to Comp Camera. then move the camera so that you can see your shatter layer off to the side. set your shatter’s render setting to Wireframe+Forces, and you should see a blue sphere wireframe in contact with the layer. the blue sphere is the area of influence of the shatter’s explosion force. any part of the layer within the sphere will explode into shapes.
if you look at the parameters in shatter’s Force 1, you will see position, depth, radius, and strength.
position dictates where the explosion force is within the layer’s area. so you could keyframe the position of the explosion sphere off the layer and have it move through the layer, vertically and/or horizontally.
depth dictates whether the explosion force is in front of or behind the layer. think of this as a wrecking ball crashing through a wall. you could keyframe depth so that the explosion force moves through the layer, from front to back or vice versa.
radius dictates the size of the explosion force. a radius of 0 (zero) will cause no explosion. if you place the force within the layer’s x/y space and its depth intersecting the layer, you can keyframe the force radius from 0 to some higher value to cause a shatter at a specified time. same goes for strength.
once a shatter has occurred, the resulting pieces are influenced by shatter’s physics parameters. at this point, the only way to ‘stop’ an explosion is to play with the physics parameters, or precompose the shatter layer and play with time remapping.
hope this helps.