The coach is exasperated that his players aren't reaching their potential, so training is especially strict.
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From 《红楼梦》 chapter 96 (though the image predates it in metallurgical metaphors): iron becomes steel only under great heat and hammering. The idiom captures harsh treatment that masks real care.
usage
Always implies care underneath the anger — closer to 'tough love' than pure hostility. Most often used of parents, teachers, coaches.
