Yes, that is close to how it works!
The apron is set a few million above the luxury tax. The apron and the "hard cap" are the same thing.
For many teams the apron is irrelevant. It only comes into play when a team that is over the salary cap acquires a free agent from another team using a mechanism other than a minimum contract (or the taxpayer MLE). The league was trying to prevent the scenarios of the "rich getting richer" by making it harder for teams that already had a lot of money in committed contracts to add more big-contract players.
So, if a team is over the cap and plans to acquire a player through either: sign-and-trade; using the full MLE (or any amount of the MLE greater than the taxpayer MLE), or the bi-annual exception, they become hard capped. This means that they must be under the apron at the conclusion of that transaction and remain under the apron for the rest of the year. Teams can use trade exceptions, re-sign their own players, and use up to the tax-payer MLE and avoid the apron all together. This does not prevent them from using a trade excpetion or making other moves, other than any move they make for the rest of year must keep them under that apron amount.
The Jazz were hard-capped all of last season because we used the MLE to sign Derrick Favors. In fact, almost half of the league was hard-capped last year. When you are a team that is just over the salary cap but still far under the apron being hard-capped doesn't matter so much because there are no realistic ways you'd get that much salary anyways. Where it becomes tricky/not doable is if the team is at/over the apron to begin with.
