mettre en berne, in the context of tracking motors for PV plants, is commonly called "stowage" (verb: to stow), as discussed about half-way down the page here:
http://www.powerelectronics.com/content/designing-solar-trac...

"The typical stepper motor speed range is also limited on the high side to about 400 rpm. This limit can be disadvantageous when it becomes necessary to stow trackers quickly when bad storms approach.
...
They also exhibit a wide speed range that is advantageous in stowing situations
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The electronically commutated BLDC motor has no wear-prone brushes, is highly efficient (typically 85 to 90%) and hits 3,000 rpm, a distinct advantage when a short stowing time is important."


The purpose is to put the panels in a "safe" position when not in use (power grid unavailable, adverse weather conditions, etc.), and the ST is discussing the back-up power needed to drive these motors when grid power is not available.