There are two sides: The server, and the client/browser.
When SoSci Survey creates a page, this is done server-side. Then the page is sent to the user's browser. It won't leave that side unless the respondent clicks "next". Only then are the answers sent back, and processes server-side.
However, if you would like to create an IAT, there's also a module implicit methods available for SoSci Survey. Academic theses can get a 80% discount on the regular price upon request. The SC-IAT in the package also allows for a "respond faster" notification.