Sell button:

If switch is set as Market order then instruction to Sell specified amount of a chosen currency pair at the current bid price will be sent to a dealer.

If switch is set as GTC order then Pending order to Sell specified amount of a chosen currency pair at the specified price will be sent to a dealer and placed in Dealing Desk system.

If you have pressed Quote Button and dealer have sent you prices for sell and buy, than pressing Sell button automuticaly results in making deal.