(from the spec)
7.4 Write Locks and Null Resources
It is possible to assert a write lock on a null resource in order to lock the name.
A write locked null resource, referred to as a lock-null resource, MUST respond with
a 404 (Not Found) or 405 (Method Not Allowed) to any HTTP/1.1 or DAV methods except
for PUT, MKCOL, OPTIONS, PROPFIND, LOCK, and UNLOCK.
A lock-null resource MUST appear
as a member of its parent collection. Additionally the lock-null resource MUST have
defined on it all mandatory DAV properties. Most of these properties, such as all
the get* properties, will have no value as a lock-null resource does not support the GET method.
Lock-Null resources MUST have defined values for lockdiscovery and supportedlock properties.
Until a method such as PUT or MKCOL is successfully executed on the lock-null resource
the resource MUST stay in the lock-null state. However, once a PUT or MKCOL is
successfully executed on a lock-null resource the resource ceases to be in the lock-null state.
If the resource is unlocked, for any reason, without a PUT, MKCOL, or similar method
having been successfully executed upon it then the resource MUST return to the null state.