What it does:I forgot about that command. That command often helps with fixing terrible lag when playing online. Not sure how that works. Thanks.
Refreshes the cached DNS names that are stored on your PC.
if server.bfbc2.com is located at 1.2.3.4, and gets moved to 2.3.4.5, that command will clear out the cached location of that resource and force your PC to look ip up, get the new IP, and then record the new IP.
Now, this will only work if you are using DNS caching on your PC, which, honestly, no one should really do. Your best bet is to use a non-caching set of DNS servers. Additionally, most people are behind a router, and they are pointing to that for a refrence set for DNS.
Example:
Your PC is at 192.168.1.10, your router is at 192.168.1.1.
You plug your PC into your router, which automatically assigns an IP address within that range, and the IP address of your router is also going to be your DNS server for your network.
The router might look to your ISP's settings that come automatically and assign those DNS servers to your router (or they will have you put them in.)
If allowed, use external DNS servers from your ISP. I guarantee that a good majority of ISP's will cache DNS to reduce traffic on their own network (which is laughable at best, as DNS queries are miniscule and quick, and it's all internal anyways).
OpenDNS
Cloudflare
Google DNS (which ...they do both caching and non caching)
Now, how does it speed up? Well, EA uses a CDN (content delivery) to resolve these servers. It's possible that they are not spending any time (or money) to update their records on the CDN, so you are spent looking up to see who has that updated record to connect. IF you are in Washington DC, and a CDN server in Washington, DC doesn't have it, but one in Montana does and that is further down the list, well, you are waiting for a while in order to get that new host.
All in all, the activity of flushing DNS does NOT speed up the Internet to your computer, it just says "hey, this resource potentially moved, where's it located at now?" It also has zero to do with latency to that specific resource. its only for updating locations of new resources. IF you have latency with online gaming, that is a different issue all together as that has to do with distance (hops) between your pc/network and the server you are connecting to.