Never easy as a 2D stunt, however, if you design an emitter (using the blank one available on the wondertouch site) that is a dark shard-shape with solid trail, it should be possible to make it branch off from the main stem while retaining its opacity.
If you're doing real life shooting for this, try to stay as smooth and straight as possible to simplify the overlay a bit. Add your camera wiggle later in affter effects.
Might be a good Idea to also use several dust emitters in tandem with the crack to add a bit more realism.
Hope that helps.
If you don't already have it, I really recommend the Fusion DVD for tricks on using PI and AE together. Stunning.