unquote does not execute anything.

But, lets try to expanding your macro by hand. Perhaps we can enlighten you that way!

Your intitial version:

defmodule Control do
  defmacro my_if(expr, do: if_block, else: else_block) do
    quote do
      Control.do_my_if(unquote(expr), do: unquote(if_block), else: unquote(else_block))
    end
  end

  defmacro my_if(expr, do: if_block) do
    quote do
      Control.do_my_if(unquote(expr), do: unquote(if_block), else: nil)
    end
  end

  def do_my_if(expr, do: if_block, else: else_block) do
    case (expr) do
        result when result in [false, nil] -> else_block
        _ -> if_block
    end
  end
end

In the wollowing steps, I wont use the AST, but the human readable representation of the AST to simplify things a bit.

And now lets expand Controlö.my_if(false, do: IO.inpsect true, then: IO.inspect false:

First lets fill in the gaps of the Macro:

quote do
  Control.do_my_if(unquote(false, do: unquote(IO.inspect true), then: unquote(IO.inspect false)))
end

Now lets do the unquote:

Control.do_my_if(false, do: IO.inspect true, then: IO.inspect false)

As you can see, we have finished expanding and compiling the macro. We have left a plain function call in the BEAM-byte code. Its arguments will be evaluated at runtime.

Now lets try the version of @OvermindDL1 which I won’t paste again, using the same snippet as above:

The first thing to mention is, that there is no quote in the macro definition, therefore the function is called at compile time! In that function is a quote, therefore wi will look at that now:

quote do
  case unquote(false) do
      result when result in [false, nil] -> unquote(IO.inspect false)
      _ -> unquote(IO.inspect true)
  end
end

And now unquote this:

case false do
    result when result in [false, nil] -> IO.inspect false
    _ -> IO.inspect true
end

As you can see, this version expands into a case-expression and not a function call. Also since do_my_if is called from inside Control during compiletime (not from another module during runtime as in your version), you should be able to even defp it in Overminds version.

Somewhere burried in the Macro module, there was a function or macro which was able to print the expanded sourcecode of a macro-call. That is very nice when debugging them.

That never happened