Pot Limit Omaha poker involves each player being dealt 4 hole-cards, 2 of which must be used together with 3 of the 'community cards' at showdown. This makes the hands shown down in Omaha much stronger, on average, than in other forms of poker. The number of possible hands (and draws to hands) with 4 hole-cards also makes bluffing more difficult in this form of poker.
Omaha is a game of 'the nuts', that is the strongest possible hands. This gives rise to a circumstance where you can use the knowledge that opponents do not have the nuts to launch a bluff. The 'Bare Ace Bluff' is one example of this.
When 3 cards to a flush are showing on the community card board in Omaha, a player with 2 cards of that suit in their hand can make a flush using the all-important '2 and 3' combination. If you have only one card of this suit you can not make a flush even if a 4th card appears on the board. However if that card is the ace of the suit in question then you have a powerful piece of knowledge - any flush that your opponent holds is not the 'nut flush'.
The 'bare ace bluff' uses this information to make a huge raise, representing the made 'nut flush' in an attempt to get opponents to fold. In pot-limit Omaha this is often made on the turn or after some raises on the flop - the ideal being that you can bet enough chips to put considerable pressure on your opponent(s).
There are several criteria which need to be in place to ensure that the bare ace bluff will work. To begin with your opponent must be capable of laying down a smaller flush, many players - particularly online - will not do so. This will cost them considerable money over time, however effectively negates this Omaha bluffing strategy. Value bet these players more often and save your bluffs for better occasions.
In order for the bare ace bluff to be effective your opponents should ideally have seen you make a similar large bet with a very strong made hand. If your usual tendency is to check or to bet smaller amounts when you are truly strong then your bluff becomes transparent.
Finally, make sure that the board is not paired - in Omaha you can not play flush or straight draws strongly on paired boards. The reason is that full-house hands are very common in this game due to the 4 starting cards each player receives. If nobody bets on the flop or turn - which makes a flush possible - then the bare ace bluff may be attempted against the right opponents.