A skiing man = man who knows how to ski? Or Man who skis...maybe this one works better in sentences where the person has two actions...one that describes them directly and the other action that happens because of what the person or thing is doing? The man who skis takes lessons e ery day The man who skiis goes fast. The man who skiis stopped.
Object markers 을/를 (like subject markers) are just role indicators of the attached word. They have grammatical values but don't actually contribute anything to the meaning of the sentence. So when it's clear from context that the attached noun is a direct object, 을/를 can be dropped, creating phrasal verbs (as seen in this case).
• This is also how many Korean -하다 verbs have been created, e.g. 공부를 하다 -> 공부 하다 -> 공부하다 etc.
• 을/를 will most likely be kept if the noun (direct object) is kept apart from the verb that acts on it. This is just to prevent any ambiguity. e.g.
스키를 폼나게 잘 타는 방법 = ways to ski well and in style
To ski = To "do" skiing => describing an activity (e.g. a sport, a hobby) = 스키를 타다.
To be skiing = To be gliding over snow using skis => describing a progressing action on skis = 스키를 타고있다
A man who skis = A man who 'does' skiing = 스키를 타는 남자
i.e. 스키 타기, skiing (ski-riding) is the man's current activity
A skiing man = A man who is riding = 스키를 타고있는 남자
i.e. 스키를 타고있기, being in the process (a dynamic state) of skiing => The man's current action.