Despite its name, momentum investing is arguably not investing at all. In reality, momentum investors tend to be shorter-term market participants that focus on stocks or sectors in response to their recent price behaviour. In other words, they buy assets when they have risen in price and sell assets when the price has fallen. As such, momentum-investing tends to be valuation-insensitive and is not linked to fundamentals.
Momentum investing is always a feature in markets but it tends to be short-term. In certain market conditions, however, momentum investing can have a more lasting impact on market behaviour. A good example of this would be the late-nineties technology bubble where momentum drove the price of internet-related stocks to extreme levels that could not be justified by fundamentals. Ultimately, however, fundamentals always reassert themselves as the predominant driver of share price behaviour.