ignoring some technicalities [as, forgive me if i'm wrong, but you don't seem to be a chemistry pro judging from your question ], i’ll try to answer some things:

As you may know, pH is a reverse logarithmic scale of H+ concentration. High pH = basic = low H+ concentration, which corresponds to high OH- concentration.
Low pH = high H+ concentration.

off subject i guess, one thing that strikes me as odd: ‘Eat something that separates the OH from OH-contianing biochemicals that are already in the body.’
that’s probably not a very effective way to mess with your pH. can you give me some examples you’re thinking of to discuss?
continuing
‘want to know what these minerals do that increase and decrease body pH’

alkaline simply means basic. some of the metals you are referring to, like magnesium and calcium fall into a group of elements known as alkaline earth metals. you’ll often find them as soluble metal oxides, which will form basic hydroxide salts when dissolved in water. example:
CaO + H2O -> Ca(OH)2, which yields OH- ions.

note: ‘minerals potassium, sodium, calcium, magnesium and zinc are alkaline-forming minerals’ those are elements, not minerals. sorry, i’m getting into technicalities.

but i mean, come one.. if you’re asking questions involving chemistry-related mechanisms, that’s kind of hard on me if you don’t have sufficient chem background. i’m not trying to be a ****. blah.

anyways, getting into one specific case you mentioned:
calcium from coral.
coral is mostly CaCO3, or calcium carbonate.

this compound is water-soluble and ionic. The carbonate ion, (CO3)-2, is what is responsible for making the solution basic. If you don’t have acid-base chemistry background, this can be hard to understand, but it will always exist in a solution as a mix of carbonate, carbon dioxide, bicarbonate, and carbonic acid.
It’s sort of illustrated by these equations:

CO3(-2) + H+ HCO3(-1)
HCO3(-1) + H+ H2CO3

where the H+ is taken from H2O.

if you’re wondering how an ion like CO3(-2) is basic even though it does not involve (OH-), this is because it falls under the definition of a base [bronsted-lowry], and is specifically classified as a weak base.

to understand how it’s defined as a base, one can use bronsted-lowry’s definition of acids and bases.
HA is an acid [generic model -- A is not an actual element here.], and A- is the base. notice they’re the same element, A.

HA + H2O => A- + H3O+

so here, the acid, HA, donates a proton (H+) a proton to water. H+ + H2O and H3O+ are equivalent, by the way.
stripped of its hydrogen cation, the HA acid is now A-. see something going on here? it added a H+ to the solution, lowering the pH [by definition of pH scale], while creating a A-.
interestingly, A- is a base.

the pair reaction:

A- + H2O -> HA + OH-

the A- stripped a H+ from the water molecule and generated a hydroxide, OH-, ion. the pH is raised — lower H+ concentration.

HA and A- are called an acid-base conjugate pair.
HA is A-’s conjugate acid, A- is HA’s conjugate base.

back to carbonate.

CO3(-2) + H+ HCO3(-1)
HCO3(-1) + H+ H2CO3

these two reactions both represent what was going on above, with conjugate bases on the left and conjugate acids on the right.
all exist simultaneously in solutions. [it's interesting to note that the HCO3(-1), or bicarbonate, while being carbonate's conjugate acid [and thus plays the role of an acid in this situation] is actually a base, eg NaHCO3, sodium bicarbonate, or baking soda. i guess that makes it amphoteric? [can act as an acid or a base] neato.]

note that I simplfied the above by providing H+ instead of H2O on the left hand sides.

CO3(-2) + H+ HCO3(-1)
is the same as
CO3(-2) + H2O HCO3(-1) + OH-
if that was confusing at all.

so, as you can see, the carbonate provided by the calcium carbonate [coral calcium] is in fact basic and raises pH.

another thing to consider when pondering pH in the body, particularly blood pH, is what the body is doing when you introduce acids and bases. the blood actually contains a buffering system that reduces the change in pH when acids/bases are introduced. so you may want to look into that too. [hint: the buffering system utilizes the acid-base conjugate pair i discussed above. ]

anyways, that’s one example.

i’m not really sure at all to what depth you want me to explain this and in what way. there are many things that go into a question like that.

let me know if you have math/science questions, or if you want to elaborate.

…gimmie points… and learn acid-base chemistry.