Under contract terms designed to protect it, the partner can buy the outgoing shareholder's stake for itself at the price agreed, staying silent through the auction process, and then pouncing after a deal is struck to reap the benefits of time and money spent on valuation by a disappointed third party.
"If I were a prospective bidder I would be looking very hard at any assets that would have a good chance of getting pre-empted," said Vijay Sethu, executive director of ANZ Investment Bank's Project and Structured Finance in Asia.
Bankers also say an increasing trend towards pre-emption may prompt changes to future shareholders' agreements.
Existing shareholders will tend to pre-empt if they consider the price is good, or if they do not welcome the new partner.
In some cases, shareholders have used their rights to force partners desperate to raise money to share profits from a stake sale, industry sources said.
World number three oil company Shell and its partner, US construction firm Bechtel, are selling the assets of their InterGen global power venture, worth an estimated $5 billion including debt.
Most of the 10 plants on the block have at least one other equity partner. According to a source close to one bidder, Shell is putting together a structure designed to minimise pre-emption risks, but he declined to elaborate. El Paso owns only part of each of the eight of the plants in its Asian sale portfolio, which carries an estimated value of around $1 billion including debt. Bidders face high pre-emption risks.
The US firm's biggest Asian asset is its 50 percent stake in a 1,800-megawatt power plant in South Korea. Its partner, Hanhwa Chemical Corp, is also seeking to sell its 38.25 percent stake in the plant - creating a pre-emption stalemate.
Bankers say each partner is waiting for the other to sell first, so that it can exercise pre-emption rights and then sell a majority stake at a higher price.
The number of pre-emption cases involving power, oil and gas assets in Asia has been rising in the past few years.
Chinese oil firms CNOOC Ltd and Sinopec were thwarted in their attempt to buy a stake in Kazakhstan's Kashagan field in 2003, as existing partners pre-empted UK firm BG Group's $1.23 billion sale of its stake.
Last year, Edison International's sale of its stake in a Philippine power station was pre-empted by its partner, Argentine firm IMPSA.
The enthusiasm for pre-emption reflects a scarcity of new energy projects, as well as improved financial health among energy companies. But it has frustrated buyers, bankers and lawyers because it wasted their efforts, time and money.
They say not much can be done to cut pre-emption risks while shareholder agreements carry the clauses that are designed to protect companies from being stuck with a partner they would not choose.
But active communication among partners before a sale could reduce the chance of last-minute pre-emption.
"The way to mitigate the risk is to talk to each other," said Brian Little, regional head of structured capital markets for ABN AMRO in Asia. "It is a bit like marriage."
Some bankers want to see a new type of clause that allows partners in a project to share the profit from a stake sale, rather than pre-empt. But lawyers say this may not prove popular, because companies set such a high value on having the right partner.
A system of tag-along rights, giving the remaining shareholders the option to force a buyer to acquire its shares along with those being sold, is also being proposed. But Raj Pande, lawyer at Paul Hastings specialising in projects and acquisitions, doubts this would work.
"It might be difficult ... because it effectively forces the remaining shareholders to live with the new shareholder or have to sell their stake," he said.