The purpose of an SWP is to ensure that you have a regular flow of income to meet expenses by automatically withdrawing an amount from your investment each month (or any other periodicity).
So, if you have, say Rs 10 lakh in a fund, you can set up an SWP to withdraw Rs 10,000 every month. The fund will redeem units equivalent to this Rs 10,000 and pay the amount out to you.
When you need to make steady withdrawals:
For other accrual categories such as short duration, banking & PSU debt, or corporate bond, keep investments to a third of the portfolio. These funds can pep up your SWP portfolio returns. But start withdrawing only after completing 2-3 years of holding.
An SWP does not necessarily mean that your capital will stay intact forever. Based on inflation, your withdrawal and returns, you may eventually start drawing down from your capital. Your fund units can go to zero.
Maintaining a reasonable withdrawal rate helps push back the time when you draw from the capital and lets your corpus last long enough that you don’t fall short of money. This is unlike an FD, where your principal stays intact.