I believe they're asking if they understand how it works correctly, in case they're missing something as to why it's 6 mana and rare.
Firstly, you can trade this enchantment which tends to do nothing except 'enable' trades, for your opponent's best permanent.
Secondly, because you have to exchange it with a target permanent you neither own nor control, permanents you do steal are safe (because your opponent owns those permanents, they cannot target them to steal them back).
It probably needs some building around to be used effectively, but essentially, imagine you had 6 Island
s and Conjured Currency
. Your opponent has 4 Forest
s and 3 Garruk's Companion
. If nothing else is done to the board, when all is said and done you'd have the Companions and the forests, and they'd have the islands (without actually doing each step out I'm not sure who'd end up with the Currency). In any case, 3 lands for 3 3/2s is often a good deal.
In short, it's likely at its most powerful when your opponent has more powerful permanents than you do. It should also be noted that the control change effect is permanent, so it combos somewhat well with something like Temporal Adept