good info on the greenshoe. If I understand this correctly the firm has a free trade on that is used to "support" the IPO. If the prices goes straight up (doubtful in any ipo), they can exercise and b/e on the trade due to greenshoe. IF the stock falls they can close out their 15% short position and profit on the difference...
That is how it works. But you do have one thing wrong: Many IPOs have performed well enough that at various points in the cycle green shoes have generated very significant profits for underwriters.