A stag buys shares though an IPO, with the intention of selling as soon as dealing starts.
Shares sold through an IPO are normally sold at a discount to the expected market price in order to ensure that the IPO is full subscribed.
This creates a low risk opportunity to make a profit by buying through the IPO process and selling as soon as dealing starts.
Although this is not true arbitrage, because there is an element of risk, it is a very low risk way of making money.