These are the facts I've got noted:
P sought to prevent D from marketing an alarm system which had not been authorised as required by Belgian law. D argued that the requirement for this authorisation breached EU law (under some Directive), and that this breach should have been reported to the Commission, but it was not. D therefore argued that the national law requiring authorisation was inapplicable because it hadn't been validly made.
(Basically P tried to stop D by relying on a particular law, D’s defence was that the law was incompatible with a Directive and so was not valid)
Yeh, it means that a Directive can be used as a ‘shield’ in a case between two individuals, in that you can rely (as D did here) on the failure of the member state to properly implement a Directive to hold that the conflicting national law (or which haven't been made using the right procedure) is invalid, and thereby justify your otherwise illegal action under it.
That might not be written well - I just lifted it from my notes.