Yes it is, also known as selling high and buying low. I have been very successful doing this. Every transaction does not work, but the totality has been spectacular over the long term.
Oh sure, it took 6 or 8 years, but the additions to OSB back in 2009 to 2011 time frame at under $10, when my initial buy was ~$100 circa 2004 or 2005, so a "loser", looks OK today sitting at $46, and I still have 82% of the number of shares I held at the end of 2011. Portfolio weighting year end 2011 was 5.4%, today about 10%. This "loser" has made me a $1/4M. Other well liked losers in my portfolio are TECK.B, ECA, MX, all hammered into the ground at one point, all made me 6 figures in the long term because I bought more at low prices. I like losers, they are very profitable.
The reverse, selling winners, has occasionally been useful too: I sold 2/3 of my Nortel common at $82 IIRC. Now there was some luck here as I needed money to buy a house, but I had to stare down the portfolio and decide what should go, and this choice saved some grief in the long term.
Now this occasionally loses for me as I said. I sold off my initial 4500 SNC and left a 6 figure sum for someone else to harvest, but this "error" is completely dominated by the successes.
Now you or someone else will tell me how you bought Microsoft, or Apple, or Facebook, or Google, and rode it to 50% of your portfolio and made $1M. The thing is I am stunningly unlucky in general and I have to as much as possible drive luck out of anything I do to have any chance at success. Holding something to 50% and having it stay there for decades just seems too luck dependent to me. If I were inclined to try this, I would have chosen a Nortel.
No, given one hammer, one nail, one thumb, hammer will find thumb. But with investing in a few dozen companies over decades and hundreds of transactions, it turns out that while the individual transactions are highly luck dependent, the totality of the process is highly deterministic, and it is really the only thing I have ever been really successful at.