I don't even know how a trial court can make this error.
Plaintiffs in an excessive force case aren't required to introduce expert testimony. Is such testimony admissible? Sometimes. Is it helpful? Often. But is it required? No way.
The trial court thought otherwise, and granted a motion for nonsuit when the plaintiff wanted the jury to conclude that an officer shouldn't have knocked down and then tased the plaintiff for (allegedly) no real reason. That's silly, as the Court of Appeal rightly holds.
Some things are permissibly within the lay expertise of juries in deciding what's reasonable. This is one of them.