Forex swap
Forex - is the simultaneous conclusion of two opposite transactions with different value dates, one of which closes the already open position, and the other immediately opens it. swap and exchange rate swap costs are determined at the time of the transaction. The operation is usually an extension of the open position.
Let's say you bought 500 000 eur / usd at 1.2347 August 17, 2004. (Tuesday) on the Spot conditions (ie, the date of settlement on August 19 - Thursday). August 19 will come to your account 500,000 eur and you will have written off 617 350 (500 000 x 1.2347) usd.
However, if you are working under the terms of margin trading with leverage, then you have to run, most likely, the camera number of dollars, and you do not smozheteoboytis August 19 (the settlement date) to fulfill obligations to the counterparty. So you have to extend or "otsvapovat" its position. Suppose, confident in his movement you have not closed the position on August 17 in the day and want to cover it with the next day, August 18. In this case, you do Swap operation - Tom - Spot, ie spend two opposite transactions different dates of calculations. Let's say that you are right, in its decision, and the price of Eur / Usd has really grown over the previous day, stopping at 1.24.
August 18, you sell 500,000 eur / usd (at 1.2400) on the conditions that Tom with settlement date of August 19 and buy them (at 1.2400) on the terms of the settlement date Spot c 20 August.
Since you had initially buy 17 August 19 with the calculations, and now the sale of 18 numbers with the settlement date on the 19th, then your claim of 500,000 euros and your obligations to supply EUR 500 000 cancel each other (netting). Positions also netted in dollars, but in part because of the transaction 2 (19 settlements) made at different prices (1.2347 and 1.2400). And since you have to list the 500,000? 1.2347 = US $ 617,350, and you have to list 500 000? 1.2400 = US $ 620,000, as a result of netting you to transfer the net difference in the amount of 2650 dollars.
After the swap, you have formed an open position, the settlement of which will occur within 2 working days, 20 numbers.

Let us again the next day, August 19 (Thursday), you again do not want to close a position (calculations that need to occur on August 20), and the price fell slightly (to 1.2387), then you spend a swap operation again. Selling calculations tom (August 20 - Friday) 500000 eur / usd (at 1.2387) and buy them (at 1.2387) estimates the Spot (August 23 - Monday).
As a result of the swap have formed an open position with a settlement date spaced from the current day (the day of the transaction) to 2 working days.
There is another very important point in calculating the swap, not described above (in order to simplify the explanation). In fact, the simultaneous operation Tom and Spot are usually produced not one and the same price, but in different, different from each other slightly.
For example, you are trying to extend their open position to buy. Then, you can sell the euro say at 1.2378 (calculations tom) and then at the mercy of the position 1.237760 (calculations Spot), or 0.4 points less. So you (ceteris paribus) earn just by being in position.

However, there is also another way: if you're trying to shed a position to sell and buy euros for Tom and sells on the Spot, the swap for you will most likely negative (or in other words, you pay for the extension position, bought a little more than sold).
The answer to why the price of the swap may be different (negative and positive, as well as the change) is that really makes the dealer, yaky fulfills your request.
Let's see what he does with an example. Let's say you are trying to extend a long position (buy position on the euro) and the transfer value date for example from 19 to 20 August. For the dealer, this means that he will need to give you euros (otvalyutirovat) a day later, but at the same time and dollars it will receive from you on the day after the deadline. Dealer (19 numbers) arises "excess" (in the day) the amount in euro equal to your position, and on the same day, the lack of (for one day) of dollars that you do not want to put it.
Accordingly, the dealer takes and places (enables interbank loan) amount in euros for 1 day, and attracts (takes interbank credit) on day 1 required amount in dollars.
Thus, the dealer places the "extra" EUR 500 000 at the rate of 3% per annum in exchange for (500,000? 3%) / 365 = 41.095 euros, which is equal to 41.095? 1.2378 = 50.88 dollars.

At the same time the amount of the dealer draws 500,000 x 1.2378 = US $ 618,900 at a rate of 2.5% per annum and pays for it (618,900 x 2.5%) / 365 = 42.39 dollars.
Net income for operations amounted to 50,88-42,39 = 8.49 dollar. This is the swap dollar, yaky he can give you. For many reasons (eg, accounting), the dealer can not be counted among you the money just like that, so he puts them in the ongoing price you swap operations. It is more convenient, and therefore taken in dealing.
If 1 point on the lot of 500,000 euros is 50 dollars, then 8.49 are about 0.2 points. Therefore, the dealer carries with you the operation as follows: for example, you are selling the euro calculations tom (19 numbers) at 1.2378 and buy it immediately at 1.237780 (ie 0.2 points less) to give it the most these 0.2 points and the corresponding dollar amount.
The meaning of a positive swap is that the placement rate for the currency that you buy at a position higher than the rate of attracting currency, you sell at the position.
If you try to extend the position in the sale, the dealer would you swap a negative (ie would take money from you) gave as the placement rate for dollars less than the rate of the euro attraction. Calculate the size of the swap points of their own.
Thus, the swap rates depend on the rates of recruitment and placement in the currency in the interbank market. Usually, holding the position to buy the currency on the high-stakes, you will receive a swap, and holding positions on the currency, with high stakes - pays the swap.
Why the big swap is calculated and deducted it from Wednesday to Thursday?
Because the date of the transfer position from Wednesday to Thursday you transfer the settlement date from Friday to Monday (three days).