According to their website "When we bottle our wine, rather than use additional sulphites to prevent oxidisation, we use nitrogen. We use nitrogen to protect the wine because it’s a flavourless and odourless gas, however when you use nitrogen in winemaking it compresses the flavour and feel – if you think of flavour as a big round ball the nitrogen flattens the back end of the round ball. As soon as you do the Mollydooker Shake it releases the nitrogen and the flavour ‘pops back out’ to it’s full size again."
I am not too sure if it is a gimmick or not, however I suppose if what they say is factual, unless you give it a good shake, it will be closed and restrained, therefore emphasising the level of alcohol in the wine.