Companies must always hold reserves, irrespective of whether they use accident year or underwriting year accounts.
For underwriting year accounts, the accounting rules will stipulate how long companies must wait before releasing profit. We often talk about 3 years because this applies to Lloyd's Names.
In this case, imagine that the syndicate has a big bank account, which receives all income (premiums, reinsurance recoveries, investment income etc) and pays all outgo (claims, expenses, brokerage, RI premiums etc). This bank account will run for 3 years. Hopefully, the balance will always be positive, ie the business will be profitable. But it can't pay any money back (to the Names) for at least three years, so implicitly that bank account is still holding reserves and retaining profit for three years.
Only after the end of three years will the balance be calculated and any surplus be distributed to the Names. Even then, reserves will still be held after the three years have elapsed in respect of claims that are still outstanding or still unreported.
In practice, it’s much more complicated than I have described here. See Subjects SP7 and SA3 for more explanation.
I wouldn't spend too much time thinking about this though. Overwhelmingly, companies around the world use accident year accounts.