I am NRI and I my US trading acct is linked to my foreign currency bank acct abroad.
If you are non-resident US person, then Your US broker will ask you to sign W8BEN form and will give u statement of trading acct at the year end without deducting any tax in US.
It is your responsiblity to show that in your tax file at your resident country as income from other sources from abroad (so that in future whenever you transfer the money, your can justify the source of income).
IMO, if it is not a big money, then you can always withdraw and keep it on Yr abroad acct and spend during travel or on internet purchase thru this acct. In that case, money never enters the India so no taxation. But if you want to transfer it to India then you might like to declare it in India after getting year end withhold tax statement.
Please check it with any qualified CA - I am telling what i am practicing. Due to my NRI status, lots of my activities in taxation are beyond Indian IT department's zone.
you find this link and pdf useful -
http://www.investopedia.com/ask/answers/06/nonusresidenttax.asp#axzz1aAVyCr8u and
http://www.irs.gov/pub/irs-pdf/p519.pdf)
Hope this helps
(regarding PM- you can send it on TJ. Plz chk with moderator /Traderji if there is any limit of minimum number of posts by user before it gets activated).