The old man came upstairs with great strength, his right hand 【holding】 a stick for support.
his right hand 【holding】 a stick for support.这是【独立主格结构】,由【名词+现在分词短语】构成,【独立主格结构】在句中【做伴随状语】.【伴随came upstairs】
句意是：“这个老人右手拄着拐杖,很费劲上楼梯”