In theory, although it seems nebulous to me, a QBO that is trending negative heading into/during the winter months, favors a weaker PV, and increases chances for a -AO/NAO, in general. During a Nina, a +AO/NAO tends to be favored, so I suppose the idea is that if the QBO is extremely negative, and the La Nina event is on the weak side, those tendencies can be mitigated, and the chances of HL blocking episodes increase. I will leave it to someone else to dig into the data for previous Nina winters to see if this has any validity.