A US citizen (and in most cases residents) owes US taxes regardless of where the taxable event occurs. It doesn't matter if you leave the US and NEVER come back. The only way to avoid US taxation is to renounce your citizenship (which generally requires you to already have citizenship in another country). Now if you are asking if you could "get away" with traveling to russia to do cash deals for rubbles and then laundering those funds though a bunch of offshore accounts? Maybe but that is asking "will I get caught", not "is this legal". Honestly if it is a small amount of money it probably isn't worth the trouble and if it is a huge amount of money speak to a good offshore lawyer and CPA. Tax avoidance not tax evasion.
Actually i was told that i can still keep my citizenship and only pay tax to the primary country if i only stay in US for less than xx days.
Many dual citizens do this.
Well the thread was about bitcoin capital gains. Foreign Earned Income "can" be excluded from US taxes but the key word is earned income (i.e. salary or wages). Sadly capital gains, interest, dividends, royalties, etc do not fall under FEIE. It is also possible the US has a tax treaty with another nation where foreign paid taxes reduce (possibly to $0) the US taxes owed. Of course this doesn't save the US taxpayer any money (if you owe $10,000 in US capital gains the only way you pay Uncle Sam $0 is if you paid >=$10,000 taxes to another nation) but it doesn't prevent double taxation (paying $10,000 to France and then having to pay $10,000 to US for the same capital gain).