What does BlockingCollection.Dispose actually do?

Having a quick look with reflector reveals this...

protected virtual void Dispose(bool disposing)
{
    if (!this.m_isDisposed)
    {
        if (this.m_freeNodes != null)
        {
            this.m_freeNodes.Dispose();
        }
        this.m_occupiedNodes.Dispose();
        this.m_isDisposed = true;
    }
}

and m_freeNodes is private SemaphoreSlim m_freeNodes; so it releases the SemaphoreSlim that are used internally.

Releases all resources used by the current instance of the BlockingCollection<T> class. (Source)