For anyone questioning the need for one of these movement holders to support the jewels while installing the chrono hands, You're better safe then sorry.
Can it be done without the holder that supports the jewel? Yes, it can but as Jon has previously mentioned you stand a chance of dislodging one of the jewels during the process. These chrono hands need more then moderate pressure to install them to "seat" on the pinion properly in order to withstand the inertia they receive during the re-set procedure.
Keep in mind: Used hands have already had their tubes stretched a bit and install easier then new hands. New hands Take at least twice as much force to install them, especially the chrono second hand.