Many people have made huge profits by owning shares of Tesla (TSLA -2.76%). Even those who only have a year under their belt with the electric automaker's stock have seen mind-blowing gains. Truly long-term investors have found life-changing wealth.

Ace investing pro Cathie Wood has benefited tremendously from Tesla's rise. The chief investment officer at active exchange-traded fund pioneer ARK Invest has dramatically boosted the performance of several of her ETFs by owning shares of the auto giant.

So when two of ARK Invest's ETFs trimmed their stakes in Tesla last week, it raised questions. Could the star investor be losing confidence in Tesla? Or is she just making prudent portfolio management decisions? Let's look at what Wood did and what it means for Tesla investors.

Four Tesla vehicles on a gravel lot in the mountains.

Image source: Tesla.

2 sales of Tesla stock

The nice thing about the active ETF model is that you get to see the moves that Wood is making in her funds almost in real time. The funds make daily disclosures of their buys and sales, and you can track them to see when there are changes in sentiment at ARK Invest.

Last week, ARK made two moves involving Tesla. The ARK Innovation ETF (ARKK -0.14%) sold about 137,000 shares of Tesla stock on Jan. 19, raising roughly $115 million. The ARK Next Generation Internet (ARKW 0.79%) followed up on Jan. 20 with a much smaller sale of 10,500 shares, producing about $9 million in cash proceeds.

The sales were part of a broader reallocation. For Next Generation Internet, the ETF used the money to add to positions in Synopsys (SNPS -0.90%). Meanwhile, the six stocks that the Innovation ETF bought the day it sold Tesla included Regeneron Pharmaceuticals (REGN 1.69%) and Spotify Technology (SPOT 3.22%), among others.

Still a big Tesla owner

ARK Invest's sales didn't change the prominent role that Tesla stock plays in these two ETFs' portfolios. The Innovation and Next Generation Internet ETFs still have Tesla as their largest positions, with a total of more than $2.8 billion invested in the stock across both portfolios. For both ETFs, Tesla makes up more than 9% of their respective assets under management.

It would therefore be unreasonable to conclude that ARK Invest has lost any confidence in Tesla's ability to retain its leadership role in the electric vehicle industry. But it does raise an age-old dilemma: Do you let winners run even after they make up a large percentage of your overall holdings? Or do you cut back your positions in favor of reallocating money to other investment opportunities?

Risk vs. reward

Long-term investors like to hold onto stocks as long as they can. When a company's underlying business is successful, it can generate huge growth rates in sales and earnings over periods of years or even decades. The stock usually follows suit, as investors have seen with Tesla as its vehicle deliveries have skyrocketed and it has started to generate a consistent profit.

That doesn't mean long-term investors never sell. But it usually takes a massive change in a company's fortunes to prompt a complete liquidation of a long-term investor's position in a stock -- something that fundamentally goes against the investor's thesis for buying the stock in the first place.

However, when it comes to trimming a winning stock, there's more debate. Some say it's prudent to diversify to reduce the risk of having a concentrated position. Others argue that if a stock is winning, you shouldn't mess with success.

When to trim

The question to ask yourself is this: Are you reducing your position because you have a stock you think has even better prospects for the future, or are you simply selling to lock in a gain?

Trimming a winning position in favor of investing in an even bigger possible winner can be a great move, especially in a retirement account for which you don't have to worry about capital gains taxes. It doesn't signal a loss of confidence in your original stock but rather the belief that you can do better elsewhere.

On the other hand, trimming for trimming's sake isn't always as clear. Raising cash as you search for better growth prospects isn't a bad idea, but you have to be prepared for your original stock to keep rising even as you look. Taking money from winning stocks to reinvest in losing stocks, however, often turns out badly -- especially when there were good reasons for those losers to underperform.

It certainly appears that ARK Invest's motivation for trimming its Tesla position was to redeploy capital to other high-conviction stock ideas. That's a worthy move -- and it doesn't say anything negative about Tesla's ability to keep dominating its industry for years to come. If you have another stock you think will do even better than Tesla, you might want to consider doing the same thing ARK Invest did.