Pass Along Funds
Budgeting for an Eastercon is a difficult thing, as many expenses happen over the course of the weekend, or in the short time before and after the convention, while money comes in over more than two years and is hard to predict accurately. And so a prudent Eastercon treasurer makes sure there's 5-10% of the budget that is available for last-minute emergencies ... so that could be in the region of £5,000.
If there are no emergencies, or more people join at the door/on the day, or some of the budgeted expenditure turns out to be cheaper than expected, then a surplus may be left in the accounts at the end of the convention, once all expenses have been paid, including any charity donations or other worthwhile projects that the committee decide to fund (like refurbishing the artshow display boards, or paying a sum towards the costs of running Eastercon.org and keeping the domain names registered (hint!).
One way of handling this surplus is to divide it up between the following two elected Eastercons, and so "pass it along". There is a custom that for an Eastercon to receive their share of that money, they have to promise to likewise pass along their surplus funds to future Eastercons.