The spell's components are V (vocals), so I believe that the caster could shout out something like "Fight me!", compelling the target in a duel with the caster. The target will then do a wisdom saving throw, to see if the spell had any effects on it, on failure it will be drawn to the caster.
Should the target now choose to attack someone different than the caster himself, he will find himself distracted, since he's being drawn to the caster. You could use that as a reason for the disadvantage.
Example: Paladin successfully casts Compelled Duel on a tribal Orc, pointing his longsword into his direction and shouts "Orc, fight me one-on-one and we will see who will stand last!". The Orc locks eyes with the paladin and prepares to throw his javelin to him, when he suddenly decides to instead throw it to the weakened halfling not far away from the tall standing paladin.. But due to the Orc feeling drawn to the paladin, he misses miserably.
How should the target know who they should attack? They were verbally told or in a different way (pointing a finger/weapon to someone for example). You can't have someone "cast" Compelled Duel without giving a way to show it. Compelled Duel can't use used if the user has no ways of provocation in my eyes. xD
Bear Totemic Attunement
I believe that the aura given from the Bear attunement becomes like a passive aura, since the Barbarian might look like a Bear while raging in that moment and the feeling of attacking another creature that is next to the bear, gives the attacker a feeling of a raging bear about to attack them (an intimidating aura).
Example: A male assassin was about to attack the fighter, but the intimidating feeling of a raging bear around him distracted him enough, that he failed a throw he was confident in.
How should the target know who they should attack? They should understand that the raging Half-Orc barbarian isn't totally what it seems to be, but when taking your eyes off of him, he is intimidating like a he was in a bear form.
The noble compelled by the mighty hero paladin to a duel, feels really intimidated by the aura of a raging bear that is right behind him, leavinh his back with cold shivers. At this point you should know, that you have to get away from the barbarian in order to attack the paladin or you choose to ignore the paladins words and attack the barbarian (or someone else in range).
I don't know, use whatever you think it should be used as. Either the spell not the feature force the target to attack them, in the end it's up to the DM or player on why the monster chose to attack a certain player.