Explanation/Reference:
1. When a container is "launched" on a NodeManager, there is a bunch of setup (localization, environment, etc.) that gets done before either the container or the YARN task can launch.
2. Once setup is completed, the NodeManager launches a process that launches the actual YARN task.
The parent process of the YARN task is used by YARN as a handle for managing the task.
3. Using the handle in 2, YARN can track when a process goes over it's container allocation for memory and will kill it.
References: https://blog.cloudera.com/blog/2015/09/untangling-apache-hadoop-yarn-part-1/