(each new calculated EMA-value takes the previous samples into account, so I think it needs a history of values longer than the actual "EMA-length" to be accurate)
Almost: EMA means that when calculating a new EMA it will take three factors into account: the current price, the
last calculated EMA value and a weight factor*. This does mean that for the first EMA you want to use to be accurate it needs to have build up a history of EMAs to compare the current price with.
*The weight factor is just how important the current price is VS how important the last EMA is. So therefor a higher weight like 21 (a EMA21 means the weight is 21) means the last EMA is more important and it will move slower towards the price (lag behind a trend).
So EMA trade bots calculate two different EMAs and compare them to determine if a shortEMA is moving away from the longEMA (using the threshold) and thus know when the market is in trend.
Yes, be careful with very short intervals...
I advice everyone that uses this bot to check out Goomboo thread that describes the strategy that this bot uses:
https://bitcointalk.org/index.php?topic=60501.
I quote from his second post in that thread:
"I caution against the lower timeframes because for the most part, they are just noise. The spread is so large and the market is so thinly traded that if you are trying to make money on a chart of less than an hour, it can be very difficult."
100% agreed, but I would add that Goomboo said that in January 2012, a time when the market was a lot smaller and different.