I'm not an herbalist, but I hang out with some, and I've looked up these herbs on line and in some books I have at home. Mother's Milk tea contains several of the best known galactalogues (milk production stimulating herbs.) From what I can understand of how they work, most seem to be bitter herbs that stimulate the liver, promote digestion of food, and most also promote urination. A few, like fennel, are also described as "weakly estrogenic"--I think they have effects on the body like estrogen. I am guessing that the galactalogues that promote urination and blood circulation promote let-down and circulation of all kinds.
That's why some herbalists are cautious about prescribing fenugreek, the strongest of the galactalogues: it can raise your blood pressure and also may exacerbate migraines and break-through bleeding, because of the way it promotes circulation. (Lactation consultants promote it all over the place though: compared to prescription drugs, it's a lot safer.)
Some of the herbs in Mother's Milk tea and in other teas are galactalogues because they are nutritious. Nettles (which people also take for allergies) red raspberry leaf and dandelion leaf (the last I think isn't in the Mother's Milk tea I have) are all rich in vitamins and minerals.
The advantage of taking the herbs in Mother's Milk tea is that they have been pre-balanced for you, and it's harder to o.d. on any one of them. Also, since many of the herbs are diuretic, it's good to be taking them in water! You will need to drink extra water when you take any galactalogue herb.
Check out the Kellymom website for more information, she has an entry on Mother's Milk tea and a glossary of all the individual ingredients.