The Forumlurker (paraphrased)
Can a Windrunner use a Basic Lashing on a specific body part?
Brandon Sanderson (paraphrased)
Yes, but it works the same way a Steelpusher could push on a specific part of metal. Usually, you'd just affect the whole body, because of its Identity and cosmere magic not working as well on parts of something that considers itself a unit. When you're very skilled, you can isolate a hand, like Kelsier could push on specific sections of metal.