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To generate a controlled cave-in you need to partially dig out, support, complete dig out, and disable the support of a block of rock.
First you need to determine the shape of the block to drop. Note that it has to fit completely into the target space, where it will fuse with the floor. Even a single tile of the falling block hitting a wall before that will stop the fall and cause it to fuse there instead. Trees have to be removed, as even the smallest twig will stop and suspend the falling block. Also, if there is empty space below the target floor the block will smash through, which might not be intended and can lead to various amounts of !Fun!.
Say your desired block is an L shape with 3 tile N/S, with a single E tile at the bottom, w tiles high to reach the cavern roof and completely seal off the target passage. We assume the cavern roof is flat, to avoid complications, and we also ignore the danger of critters for the time being.
The two lowest roof levels contain the tiles for the cave-in. The level above that should be dug out completely above the rock block, and to provide access I'd add extra room for easy access:
Top level. Dig priority 4:
WdWWWWWW
W......W
W.h.h..W
W.h.hh.W
W.h..h.W
W.hhhh.W
W......W
WWWWWWWW
first block level, dig priority 5
WiWWWWWW
W......W
W.hhh..W
W.hWh..W
W.hWhh.W
W.hWWh.W
W.hhhh.W
W......W
WWWWWWWW
Second block level, breaching into the cavern. Dig priority 6
WuWWWWWW
W......W
W.hhh..W
W.hWh..W
W.hWhh.W
W.hWWh.W
W.hhhh.W
W......W
WWWWWWWWW = Wall
d = Down stair
. = Open floor
h = Channel
i = Up/Down stair
u = Up stair.
When the top level has been dug out, build a Support on top of any of the tiles in the "island" and hook it up to a lever. Once the lever has been hooked up you channel away the "bridge" at the top level so the block is now fully supported by the support. Get your dorfs out of the way and pull the lever, and the whole chunk of rock fill fall down and fuse with the cavern floor below.
I actually use another level of rock to not having to worry about cavern critters. I shift all the stuff above up one level and then dig out a floor below the lowest one (i.e. an open 7*6 floor below the hanging rock segment). This no part of the digging process is open to the cavern, and thus completely safe (apart from your own mistakes and dwarves total disregard for safetly, of course). When the lever is pulled the cave-in will smash through the floor below, and if the cavern is two tiles high, it will fuse with the ceiling rock as well, thus sealing the hole it just made.
The extra walk tile around the channel is strictly not needed (apart from when you channel down to the cavern, where the dorfs will need somewhere to stand), but are a remnant from when I dug floors first and then channeled.