He fell off the ladder that his wife was 'stabilising'. As you know, or should know, there is nothing gradual about falling off a ladder, and the automatic reaction is to tighten your grip on whatever you are holding, in his case the chain saw power lever. Right on top of wifey below.
Although it was a hedge trimmer, not a chainsaw. Those have gaps between the moving and fixed blades which are only about 1-2" wide, so I'm not sure how her neck fitted into one of those gaps for her to be decapitated. Even nicking her carotid artery up the side of her neck might be pretty difficult. Amputating fingers is a different matter.
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