No spring needed!
OK... look at the mechanics of a turning cylinder...
(I'll consider only One BB cylinder - the horizontal one - the other should work the same)
BB#1 contacts the outer cylinder-- BB#2 contacts the key.
When an improper key is inserted... BB1 is forced into the outer cylinder... obstructing rotation.
When the proper key is inserted... BB1 may be in contact with the outer cylinder... rotation of the cylinder will apply pressure to move it toward the key... fitting a detent provided for it.
(Ya know... researching their patents may give this kind of info, too!)