Need help understanding how to profit from bitcoin trading (like forex?) I want to understand something very basic. How does trading work between BTC &amp; USD. 

Let's say I have 1 BTC at US 100. The naive idea is "buy low" and "sell high". But I don't grasp how that work, ie, how is possible to profit in both directions.

If the value of BTC change this way (value in USD):


  110 85 102 40 70 90 115 130 100 200


What I need to do to turn a profit? And if my plan is that I don't lose more than I invest at the start?
Pretty simple. You just make money on the differences when you buy/sell (taking in to account fees).  

In your example, you start with 1 BTC purchased at $100 USD.  Assuming no fees, you would:


Sell at $110 (Profit: $10)
Buy at $85
Sell at $102 (Profit: $17, total: $27)
... and  so forth


Now, you could get even more creative, since you can purchase fractions of BTC.  In this case, you could:


Sell at $110 (Profit $10)
Buy 1.294 BTC at $85 each for $109.99
Sell at $102 for $131.98 (Total Profit: $31.98)
...