Ocsigen is not using the usual preemptive threads model to handle simultaneaous requests. Instead of this, it is based on a model of cooperative threads. Ocsigen's cooperative threads are implemented in monadic style by Jérôme Vouillon, which makes them really easy to use.
If you want to write an extension for Ocsigen or a Web site with Eliom, you must use this thread model.
See Lwt's documentation.