I remember back in 2005/2006 when I was still touring the APAC circuits such as Sydney (Australia) and Kuala Lumpur (Malaysia) doing training and consulting gigs for customers, partners about Windows Workflow Foundation (WF) and Windows Communication Foundation (WCF, previously - Indigo) and some of the initial Windows Workflow questions came up regarding the use of Parallel Activities. It came as a surprise to many people that parallel activities are not independently asynchronous.
I explained that a WF instance gets only one instance from the runtime. There are reasons for this single-threaded execution model so each activity have to work with this single thread efficiently. There are ways to spin off differents thread when real parallelism activities are reqquired but because documentation was scare at that time, I had some trouble articulating how to do so.
I just read "Multithreaded Parallelism in Windows Workflow Foundation" on MSDN and while it is a definite deep technical article, if you can grok it, you will understand how "MultiThreaded Parallelism" can be done in WF using both the (rather hard-to-use) "Call External Method Activity (CEMA)" and the "Handle External Event Activity (HEMA)". Not only that, the authors (whom actually implemented such a system for their own use) also shared how to pair those 2 activities up using correlation and how to create wrappers aoround them so that it can be reused and therefore "not require talented software developer use of call-external-method and handle-external-event activities along with the CLR thread-pool"
A gem of a read.