That's the result of a Hohlschnitt. I haven't been able to find a term relating to a special trimming method but this is what a Hohlschnitt will produce.

French cover is the term for the whole bookbinding process rather than a description of just the spine.

Hohlrücken
Einbandrücken, der nicht auf den Buchblockrücken aufgeklebt und nur an den Gelenkstellen mit dem Buchblock verbunden ist. Er wölbt sich beim Aufschlagen des Buches nach aussen. Es entsteht ein Hohlschnitt.

http://www.frigg.ch/bbb/glossar.htm

French cover - The origin of the French binding method is an antique way of bookbinding, and has been evolved over many centuries. In the old bindings the leather of the spine was directly attached to the text-block. With the French method, the spine remains detached from the book, thus avoiding frequent bending of the spine and damaging the gold lettering and tooling. French binding is very elaborate and time consuming, and only valuable books are bound this way, mostly with a full or three-quarter leather cover. It is impossible to mechanize this technique.

http://www.dapilade.it/6.0.bookbinding/4.2.glossarybookbindi...