This year I have broken the golden rules of investing I’ve lived by since entering the world of finance nearly 30 years ago.With most of my savings I continue to invest sensibly for the long term, buying and holding for decades rather than days, and spreading my risk over a wide range of investments.I even set up a business called Boring Money – the name tells you everything you need to know about my general approach. But I’ve also gone a bit wild on the side. And, much as I hate to admit it, it’s paying off. My bit on the side pot has made me more than £9,000 this year after deploying a strategy called momentum trading. This is where you buy investments that are seeing strong and continued price rises and then sell with the aim of making a quick profit – hopefully before the trend fades.You won’t ever find the optimal time to buy and sell, but you can try to ride the wave, benefit from growth and get out before it falls.Think of it as a train. As it sets out from the station it gets faster and faster. And when it stops, it doesn’t come to a halt instantly, but rather starts to slow down. The same patterns can be seen in some financial markets – and you can use that momentum to your advantage.It’s risky because timing the markets is notoriously difficult. But I find that it is easier than usual at the moment because of the exuberance in financial markets – they do see occasional dips but overall they keep going up and up, fuelled in part by social media and investor hype. My bit on the side pot has made me more than £9,000 this year after deploying a strategy called momentum trading, writes Holly MackayThis year, markets are running hot and worrying better mathematicians and economists than me. The share prices of top US tech stocks such as Nvidia, Alphabet and Meta have had a wonderful run for years. Semiconductor companies have been booming. The US already makes up over half the value of stock markets around the globe – yet continues to grow. And shares in Elon Musk’s SpaceX recently went public at a bonkers valuation.One strategy I use is to look out for a popular share or index showing momentum that has just seen a fall of around 5 per cent. I jump in while the price has dipped and invest, wait until I’ve enjoyed a 5 per cent return, then sell and take profits. On occasion I’ve done this on repeat with the same volatile asset.I use an investment platform that does not charge for trading shares, so I am not losing chunks of my profits to fees. Platforms that operate in this way include Trading212, Freetrade and InvestEngine.When valuations are continuing to rise, it’s tempting to hold on for even more. But I keep banking profits as I go and selling out pretty quickly. That way I won’t get too stung if there’s a sharp downturn.For example, I bought shares in SpaceX ahead of its launch on to the stock exchange. I think SpaceX is overvalued, but for momentum investing that’s not what matters.My view is not important. What does matter is that I was confident that other investors would be hungrily buying shares to get in on the latest trend and that would push the price up, at least in the short term, so I could sell my shares for more than I paid for them and bank some profits. Momentum theory can also be fuelled by what is rather brutally referred to as the Bigger Fool theory. This is that all you need to make money is for there to be a bigger fool than you waiting in the wings to buy at a higher price.I sold my SpaceX shares early, before much bigger investors did the same thing. I took a profit of more than £2,000 in a few days, then ran for the hills.I will consider doing something similar when AI giant Anthropic launches on the stock exchange. So I trade. It’s my naughty secret. But I do it minimally and with respect. Momentum is powerful. On the up it’s amazing. But when it turns, it hurtsI have also traded Micron Technology, buying high and selling higher as explosive AI demand drove this stock up. Oil too. The price has been jumping around like a yo-yo against an uncertain geopolitical backdrop. And the healthcare sector is showing strong momentum, with returns fuelled by anti-obesity drugs and other pharma.Momentum presents opportunities for more confident investors. Rather than ignore these and preach abstinence, I’d rather suggest some vital rules of engagement.This is not a long-term strategy – it’s speculation, and certainly not for beginners. It should be a small bit on the side, not the main event. Don’t do it if losing the money would cause you financial stress.Set strict limits on when to sell before you get emotionally involved. Take any profits quickly. Don’t be greedy. Read up on ‘limit orders’ and ‘stop losses’. This is where you set price limits to avoid uncapped losses and stopping you from getting too greedy.Momentum trading tends to work when markets are rising overall – at other times it’s a different story. At the moment we’re in such a time, but it won’t last for ever.The MSCI World Momentum Index (a collection of the world’s winning shares of today) is up around 35 per cent in the past three months, more than double the standard World Index which is up by nearer 14 per cent.So I trade. It’s my naughty secret. But I do it minimally and with respect. I have never got over seeing a suited man sobbing painfully on the steps of the Australian Stock Exchange in 1999 as markets crashed around him. Momentum is powerful. On the up it’s amazing. But when it turns, it hurts.