First: We prepare commands for a voluntary action. This is done in the pre-motor cortex for actions that are reactions to external stimuli and the pre-supplementary motor cortex for ‘intentional’ actions. These commands are signaled to the motor cortex.
Second: The motor cortex executes the commands.
Third: Both the pre-supplementary motor cortex and the motor cortex sent signals to the parietal cortex, where the sensory consequences of the motor command is predicted. The pre-supplementary motor cortex signal give the sense of an urge to move. The motor cortex signal is used to create a prediction in the parietal cortex, the very near future projection of the movement in enters consciousness.
Normally, the conscious experience would be of a movement preceded or not by a sense of an urge to move.
However under the experimental conditions. If the movement is produced by direct stimulation of the motor cortex in a way that bypasses the signals to the parietal cortex – the movement happens but there is no consciousness of it as the prediction is not made. If the parietal cortex is stimulated directly than the urge and/or the movement are made conscious without the movement actually happening.
So the result is not so odd after all.
The question of how free is free-will now is a question of the difference between the way that the pre-motor cortex and the pre-supplementary motor cortex initiate the creation of a motor command.