I think they say just to invest 5-10% of your trading capital for a particular trade before doing that you have to calculate the market range where your chosen currency is fluctuating then you have to judge and see the number of lots you can open according to your trading capital . and money management is important in this business and a successful trader must learn it and try to use it during his trade. this business is nothing without learning and learning very well in this business and this business is very dependent on learning.