I am trying to update this tutorial on implementing Facebooks BigPipe to razor.
There is a html helper extension that adds a pagelet to a list, and then outputs a holdin
A Razor view is rendered inside-out. Basically it writes content to temporary buffers which get written to the response stream when the top most layout page is reached. Thus, writing directly to the response stream from your HtmlHelper extension, will output it out of order.
The solution is to use:
helper.ViewContext.Writer.Write("");