It involves comparing the relative costs of two potential policy mistakes in the current uncertain environment: tightening when such a move turned out to be inappropriate and failing to tighten when a tightening would have been appropriate.
The maximin solution (patterned after the solution to the “prisoners’ dilemma”) is to select the option that would yield the smaller cost if the policy turned out to be a mistake. This analysis does not, in this case, help one to understand why the policy action was taken in March, rather than earlier.