Method of checking collision below a block?

You can use Region3. wiki: https://www.robloxdev.com/api-reference/datatype/Region3. Here is something that should work for a single tetris cube:

local tetrisFigure= workspace.TetrisFigure --change this into the complete model of the falling tetris figure
local cube = tetrisFigure.TetrisCube -- change this into a single (falling) tetris cube in the tetris figure.
local reg = Region3.new(cube.Position-Vector3.new(0,cube.Size.Y+0.1,0),cube.Position-Vector3.new(0,cube.Size.Y+0.1,0))
local parts = game.Workspace:FindPartsInRegion3WithIgnoreList(reg, {tetrisFigure})
if #parts ~= 0 then
    --a part has been found under the cube
    --stop the cube
end

NEGATIVES:

1. you have to loop it constantly.
2. you have to do this for every single cube in a complete figure.

POSITIVES:

1. it has an ignore list, so if there are parts that you want them to fall through, just add it to the list.
2. For me, region3 is accurate, and I haven't seen a region3 error for no reason at all.

-I haven't tested it, but it should work, if anyone finds an error comment it so I can fix it.