How to use the getConnectedParts function?

No need to use the messy getConnectedParts() method!! Just use the Touched event. Here's an example

local blockA = game.Workspace.blockA --Set this to blockA
local blockB = game.Workspace.blockB --Set this to blockB
local val = game.Workspace.ValueThing --Set this to value to decrease
local decrement = 1 --Set this to how much the value should decrease by

function decrementValue() --Define a function
    val.Value = val.Value - decrement -- Decrease the val by decrement value
end --End function

blockA.Touched:connect(function(hit) --When blockA it hit, this will fire
    if hit == blockB then --Check if the part blockA touched was, in fact, blockB
        decrementValue() --Call function to decrement value
    end --End if
end) --End handler function

[EDIT] I edited this so that the decrementing could be a single function to be called anywhere.