Index funds follow a passive investing model. When you invest in index funds, the fund manager invests the fund’s corpus in the same securities and in the same proportion as are found in the underlying index.
Indices are re-balanced and re-constituted periodically. In line with this, the fund manager also re-aligns the fund’s portfolio composition by selling outgoing securities and buying incoming securities of the index.
Through this strategy, index funds look to earn returns similar to those returned by the underlying index, subject to tracking errors.
For example, a NIFTY Index Fund invests in stocks of companies that make up the NIFTY 50 Index in proportion to their weights in the index. The objective is to achieve a return equivalent to the benchmark NIFTY 50 Index. For instance, if Tata Motors has a 14.6% weightage in the NIFTY 50 index, the fund manager of the NIFTY Index Fund will allocate 14.6% of the portfolio to Tata Motors stocks. The same proportional allocation will be maintained for other stocks in the index.
In a passively managed index fund, the fund manager replicates any changes that happen in the index. If a stock's weightage increases or decreases in the index, the fund manager also adjusts the holdings in the index fund portfolio accordingly. Similarly, if a stock is removed from the index and replaced with a new stock, the fund manager sells the removed stock and purchases the new stock in the same proportion as it appears in the index.