The easiest way to "see it", is by playing with WinISD, although, if you've been a musician most of your life, but never really messed with speakers, it'll be pretty hard to translate what you're seeing into what you'll be hearing, ar least until you've had a couple real-life examples to sort of calibrate your ear.
You can think of it as a bit of built-in eq affecting the lower end of the speakers range but it does a good bit more than that. Like making the most of the drivers lowend extension (in combination with the box size), control the cone on low frequencies (and then let go of it at even lower frequencies, ie: "unloading"), and get the most of it's displacement and power handling capabilities.
Couple thing it doesn't do.
A. Does virtually nothing above 200-300 hz, they all act like a sealed box above that, it's all about controlling/shaping the lower end of things.
B. Has nothing to do with the note frequencies. Example, you don't tune a box to 41hz because you'll be playing an E.
It may seem backwards at first, but, a higher tuning increases bass response...or what people call "midbass". The area around 100-120hz and some on either side of that, depending on the driver/box size/total system. Tuning high makes the speaker sound louder/bassier. Too high can lead to boom, especially in bad sounding rooms, lack of distinction and clarity to your notes, and in the worst cases, even screw up your sense of pitch on the low notes. There becomes too much of those boomy sounds and not enough upper harmonics by comparison.
Tuning lower can reduce the low bass response. You do that to remedy some of the boom problems, especially in smaller boxes. Too far that way with the wrong driver/box combination and you can start to suffer lower displacement limited power handling and a generally thinner sound.
Another trick is to tune very low...lower than may seem right at first. This can cost you some power handling, but it makes the response curve/lowend rolloff start higher up, but gives it a shallower slope. This can mimic more of a sealed cab or "vintage" sound. Having the bass roll off a bit higher, but not so steep where the bottom just falls out after a point, accentuates the lowmids by comparison and gives a fat, but not overly deep, "classic" sound.
Pretty vague descriptions there I know, but it's about as good as I can do until you get some time spent experimenting with this different stuff.
For an example or two.......If you've ever played an older ported cab that sounded loud, perhaps a bit boomy, but started to distort on the low notes of a 4 string and couldn't handle a 5'er well at all. That's a combination of the drivers just not having the performance the newer ones do but also because a lot of those cabs were tuned too high. The designs were pretty much seat-of-the-pants, no computer modeling, etc. When you start playing stuff lower than the tuning of the cab, the cone loses control fast, excursion goes way up in a hurry, and they can handle very little power in the lows.
A lot of this happens at the first overtone on electric bass, not the fundamental. More on that here....
http://www.talkbass.com/forum/f15/bass-frequency-waterfall-plots-what-they-mean-rigs-510749/
Another good read is parts of fEARful threads/users impressions of the shelf port extension and how it changes the sound. That's an example of using a lower tuning to reduce a bit of bass output to give the cab a shallower/smoother rolloff.
FWIW, I tune mine to have about a 2db lift in the lows before they take the dive towards the bottom. It isn't right or wrong, just suits me. Our best paying gigs are often outdoors, sometimes at smaller courtyard/patio/winebar type affairs where my rig can carry them on it's own. It's just enough extra to keep a full sound while playing outside while still remaining clear/even, whether for the whole crowd, or when it gets over 100 people and I use the bigger PA, on stage for the band, without having to boost lows, lose headroom, or have to deal with a thinner sound than I like. Your situation may be completely different.
Whether you're experienced or not, your builder should be. Some simple tone descriptions of how you like your sound and what type of music/type and size of gigs you play should be enough to lead him in the right direction as far as how to make your lowend sound/what to trade off for what, etc. At least close enough to get in the ballpark and maybe make a simple adjustment to get you all the way there. If you specify Reggae and he sends you something that sounds like Rush, he's not doing his job. Something less extreme, at least in the ballpark, he's trying to turn your tone descriptions into a physical thing. A lot of how the lowend sounds isn't just the lowend alone, it's the lows in comparison to the rest of the bandwidth. With a regular 215, not much you can do about that without some intricate filtering work, when it comes to 2 and 3 way systems and more involved crossovers, you can make it sound like pretty much anything given enough time and expense.
Maybe somebody else can give clearer descriptions but it's good to ask. More informed questions get better answers and better results.
! Yes, of course you're right. And I'm sure he is. I'd just like to not be at 100% ground zero when I'm talking to him about tuning, or probably much more correctly, listening to him tell me about the frequency & porting/venting he thinks is best. And if I have a question I'd at least have a chance to make a little sense when I ask it.
! Thanks! Trying to digest.
