The FTP feature found in the settings of yoflash is for advanced user only. It is stil in an early stage and completely experimental. It is designed to copy the games on a different server (or a subdomain) inseatd of your Joomla webpage in order to use this server for a Content Delivery Network (CDN).
A content delivery network or content distribution network (CDN) is a large distributed system of servers deployed in multiple data centers across the Internet. The goal of a CDN is to serve content to end-users with high availability and high performance. [Source wikipedia]
In order to use this feature, you have to have a second server, or a subdomain with enough space to copy the flashgames to. Let's say, we have a website on www.foobar.com and you created a subdomain called cdn.foobar.com (or games.foobar.com). You will have to setup the FTP properties and teh domain names by yourself . This is usually done within the administration interface of your hosting provider and is different for every provider. Please ensure that FTP is enabled and you may want to tru to access your cdn space with programs like WSFTP or FIleZilla first.
In our little example, I have created a subfolder (cdn.jingames.com) and given it a seperate FTP User and Password in my server's administration panel. You can also see, in that folder I created another folder called 'games'. This will be my base-folder. I want all games to upload to cdn.jingames.com/games.
Once you ensure that it is working, tell YoFlash that you want to upload files to FTP. By default this will only upload the game files. You can also upload the image files as well, however, this is NOT recommended as it may slow down your website. For a flashgames it is not so bad when it doesn't load with full speed, but consider you have 50 images that load slower then they could do (i.e. in a category view) then this might be an issue.
And that's it. From now on, the games will be copied to you FTP server instead of the original Joomla /images/games folder.
Migrating: If you want to switch from normal to FTP mode after you uploaded games already, you will need to copy the whole images/games directory to your FTP server.
Now, you can actually set up your CDN (i.e. from Cloudflare or oters) to point to cdn.foobar.com. In Cloudlfare this is done by changing the DNS settings of the whole domain. Again, this has to be done in you server's / domain's administrator panel and can be a pain sometimes, as it requires you to wait for a certain amount of time, befor every DNS server in the world has been informed that your data has been changed. The I activated cloudlfare for the cdn.jingames.com but not for jingames.com (or www.jingames.com). The picture below shows, how my settings in cloudflare looked like: