As far as I understand the snap off boss has both the boss that fits on the steering column and the one that fits on the steering wheel. they mate together and you have a key so you can lock and unlock the wheel depending on whether you want to take it with you or leave it in the car.
You remove your old wheel and fit the new boss, it then has a cover that fits over it, with a wire you have to run from the horn wiring that makes contact on a brass connector on the boss. This is secured with allen key bolts and you can then put little dowels in the tops of them that will need drilling out if you ever want to remove the boss.
Also I've heard that it sits the wheel closer to you than the original wheel but I suppose you'll get use to that.