Beginning with 2017 Week 42, Differential has a new "draft" state for revisions. Currently, this is only active if prototypes are enabled, although it will become a default behavior in the coming weeks.
When revisions are created, they are initially "Draft" revisions until builds pass. Once builds pass, they are submitted for review. (If you don't use Harbormaster to perform builds, all builds automatically pass immediately, so this state is transient.)
You can share the URI to a draft revision normally, but it won't appear on reviewers' dashboards yet and they won't be notified via email or notifications that it exists. Once builds pass, it will automatically promote to "Needs Review", notify reviewers, and appear on their dashboards. This raises the signal-to-noise ratio for reviewers by keeping revisions with build failures out of their queues, puts more pressure on authors to be responsible about tests, and puts more pressure on everyone to write reliable, fast tests.
You can manually "Request Review" to submit a draft for review before builds complete (or if builds fail).
In the Arcanist experimental branch, you can also use arc diff --draft to hold revisions as drafts indefinitely. You can use this as a preview if you'd like to look things over before submitting a revision manually.