Clare's right here.

Theoretically, the ideal way of shampoo'ing a carpet is to:
1. Spray an enzyme pre-spray onto the carpet
2. go over it with carpet soap first
3. and then do a second pass with an acidic rinse.

What's an enzyme?

And enzyme is simply a protein that breaks down other protens. Proteins aren't just found in meat and egg yolks; they're found in almost every food stain and many bodily fluid stains such as blood and saliva.

The idea of putting an enzyme on the carpet is to break down all those proteins into smaller molecules so that they're more readily soluble in water. You're supposed to leave the enzyme on for 15 minutes, but that instruction is designed just to give the carpet cleaner sufficient time to set up his equipment. There is no harm at all, and some good in leaving the enzyme on for longer. Once the carpet dries up from having been sprayed with enzyme, leaving it for longer won't do any good.

The reason for the acidic rinse is that soap tends to be basic in pH and most foods tend to be mildly acidic. So, acidic food spilled on a carpet with some residual soap in it will tend to cause the food to stick better than it otherwise would.

The idea behind the acidic rinse is to neutralized that alkalinity of the carpet caused by the residual soap, and even leave the carpet slightly acidic. That, theoretically, will result in mildly acidic food spills not sticking as well to the carpet.

You can buy acidic rinse concentrates at any janitorial supply store, but putting one to two cups of vinegar per 5 gallons of rinse water in your solution tank before doing the final pass will work equally well.

NOW, there isn't a carpet cleaning contractor in North America that will do a final rinse unless you pay extra for it. What they'll do instead is say:
"Not necessay Ma'am. I'm using a friable soap".

What's a "friable soap"?

A friable soap is a carpet detergent that, under laboratory conditions, dries to a brittle film on the carpet pile so that normal vaccuuming will remove the soap film from the carpet pile.

The idea here is that once the carpet is dry, normal vaccuuming will agitate the carpet pile to cause all the flakes of residual soap to break off and be sucked into the vaccuum cleaner's filter bag. CUZ, that's what happens under laboratory conditions when they test that soap.

But, when you have who knows what spilled onto your carpet over the past 6 months, those aren't laboratory conditions. I won't believe that any soap is truly friable until they test that soap under real life conditions where they pour the soap onto carpets with dried up milk, orange juice, vomit, urine and beer in the carpet as well as the usual amount of dust, pollen and road grit. THEN, if the soap dries to a brittle film that breaks off the carpet pile easily to be sucked into the vaccuum cleaner bag, I'll reconsider.

The way it stands now is that it's best to clean your carpet with the above three step process, do a second pass with an acidic rinse, but no one does that final acidic rinse unless you pay extra for it. But, at least most professionals don't use so much soap that it creates problems for the home owner, wherease that could easily happen with homeowners that shampoo their own carpets with rental machines.

Anyhow, now everyone who's kept up with this thread knows more about shampoo'ing carpets than 99% of home owners.

PS#1:
One GALLON of carpet shampoo'ing soap should be considered a lifetime supply for your typical home owner.

PS#2:
If you live in a house with an asphalt driveway, be aware that a dark discolouration on the carpets near the door you come into your house is actually asphalt tracked in from your driveway. I have an asphalt parking lot and when I shampoo the hallway carpets near the back door of the building (where the parking lot is), I use Simple Green as the detergent in my solution tank, and it works better than carpet soap or Mr. Clean at removing that asphalt residue from the carpet.