DoS (denial of service) testing takes many forms, and has many different purposes.
DDoS vs DoS
The first thing to understand is the difference between a DoS attack and a DDoS attack.
DDoS is a distributed denial of service attack, and the wording is overused, but it typically means a very large scale but relatively dumb attack. Dumb in that it's spamming traffic at your systems designed to saturate the internet connectivity, routers, switches and other devices in the network.
It is uncommon for a normal site to have to deal with sustained DDoS attacks and typically needs to be mitigated by an upstream CDN or DDoS protection provider.
DoS on the other hand is very similar but executed in a very different manner. It's usually a "smarter" attack, using web requests to generate "legitimate" traffic on your website that is harder to just block. It causes high load on your dynamic website, your databases, caches, and other services.
DoS is what LoadForge is excellent at - simulating very large volumes of requests.
How it works
To simulate a DoS attack you could create your own simple load test. For many sites a 200,000 user load test is basically a denial of service attack.
LoadForge offers specific DoS testing though which is designed to bypass caching and standard amplification systems and cause maximum load on your web servers.
LoadForge can simulate hundreds of thousands of users, requests millions of requests a second.
You can scale up your DoS testing as you go, and monitor your servers and the health and performance of your systems.