This is like saying buy low, and sell high...no ****, but easier said than done...
I would almost bet if you followed this strategy you would get crushed... the DOW has been going up since January, would you buy it now? I certainly wouldn't. It is in a topping pattern, and volatility is entering, and close to a double top, I would want confirmation and a 2 closes above 14200 before I would go long the market right now. I am actually looking for an opportunity to short the market.
There is a lot more to trading than that statement - but it is taken out of context so I will reserve judgement.... who wrote the book?
Ok let's assume a stock is going up and you buy it. What makes you believe that it will continue and that it isn't the top? Also based on this if this isn't the top you don't know where the top is. It's very easy for authors to make vague assertions like well you need to buy low and sell high but the reality of trying to do this is very different.