That will be a real mess.
There
was pressure at the impact point but it is unlikely that would cause a long, wide split unless a second nearly simultaneous injury occurred at or nearly at the same time, causing two fractures to join.
You might also want to try hitting a coconut with a hammer just enough to cause a depression and fracture. Then put the coconut on the counter and press down with your hand, putting your weight behind it. The fracture will widen and lengthen (or at least it should!!!) because of the relatively high pressure on the coconut (and other reasons but I'm too tired to get into that again tonight
) Then try hitting a second coconut with all your might and see what the hammer does and how it fractures. It is a rather gruesome experiment but it helps understand JonBenet's wounds.