It's simple in its principal, really..

• you have multiple companies
• one company (A) have some supply for some product
• on the other company (B), you create demand for that product (i.e. for its operation)
• thus, under your control, you make company A and B enter a trade agreement
• as someone who brokered that deal, you get rewarded (e.g. from brokerage fee, or commision)
• sometimes, by having a massive increase in sales, the stock for company A would increase, thus you can sell a little bit of it, which you can later buy back after the stock price goes back down
• profit

Some facts:

• even though they're your companies, you are a separate entity from them, and they are each its own entity
• the money comes from the investors as well as profit, remember that they are separate
• no, you cannot just take all the companies' money, since even though they're yours, there are corporate structures and other people at stake preventing that

So you basically come up with some excuse for moving stuff around, then you come up with some excuse to siphon off some of that good stuff.