Uh, I can't get it to work. Part of it is having 1 in $03AF, which gets written there only at that place, since the big stone that gets broken is in slot 0x1C, and it's descriptor at $03AF (that looks like sprite timer) becomes 1 if there are enough shots on the screen.
When the huge rock is crushed, small stones, if shot, have their timers going 4 to 1 in loops, but when there's enough lag, most of that is skipped, routine gets interrupted by PPU stuff, and 1 sticks in that address forever, while the stone itself isn't crushed.
That 1 is then used by Y register, when stuff is written to adresses $0460+. When Y is 1, 0 is written to $0461. There you go.
So basically, the whole trick is trying random shit to get these 2 during the same few frames. I could get that 1 there, but the rock only crashed into ONE piece, and then nothing cool happened.
http://tasvideos.org/userfiles/info/12852061334348625