Reference to an attack Goku and Vegeta did during the Buu saga.

I stopped watching before Buu showed up so maybe you're right, but this looks more to me like Goku helping Gohan obliterate Perfect Cell.

Good job, Past Me! Time to split!

DoctorVisigoth said:
Good job, Past Me! Time to split!

You win a few Internets for that reference.

Tetrominon said:
I stopped watching before Buu showed up so maybe you're right, but this looks more to me like Goku helping Gohan obliterate Perfect Cell.

That's what I thought at first but then I realized both Reimus are using both hands, instead of one of them using only one hand.

It looks like the PC-98 Reimu is older than the Windows Reimu, so I'm guessing this is intended to be a "PC-98 Reimu is Reimu's mother" headcanon, particularly since this is a parody of the Father-son Kamehameha.