The ExplosionStart Event is fired shortly before an explosion starts. If canceled, the explosion will not happen.

You will need to cast the event in the function header as this class:
crafttweaker.event.ExplosionStartEvent
You can, of course, also import the class before and use that name then.

ExplosionStart Events implement the following interfaces and are able to call all of their methods/getters/setters as well:

• IExplosionEvent
• IEventCancelable