In Freestyle job, the "Trigger parameterized build on other projects" step in Post-build Actions seems to block an executor until the triggered downstream job starts. This creates a situation, where using this plugin might lead to deadlock, which can only be resolved by cancelling the step. 
For instance, I have 4 available executors. I schedule 4 jobs to run at the same time, all of which have a parameterized trigger in post-build actions. Once any job is finished, it attempts to trigger its downstream project, however since there are no free executors available, the downstream project is simply moved to the build queue. This prevents the upstream project from ever releasing the executor, which results in a deadlock. I'm not sure if this is intended behaviour, however this makes the plugin unusable when applied to multiple projects. Here's what it looks like: