How does Sayyidul Istighfar work?

There must be a sense of sadness, a sense of remorse about what we have just done. If we haven't felt anything in our hearts, the hearts are hard. We sought refuge from the hard heart, the heart which had no fear. If there is no sense of sadness, a sense of remorse about what we have just done, that du'a, sayyidul istighfar, no matter how powerful it is, will be of no benefit. So, we have to be sincere in this.

If we seek refuge in Allah from that sin, he will give us refuge, but we must be sincere. A part of sincerity is giving up that sin. It doesn't make sense when some people smoke cigarettes, and they know it Haram, some do say that it is makrooh, but in reality smoking is Haram. They know it is Haram. They feel bad and then ask Allah to forgive them from smoking and they are still smoking. How is that going to work? They have to give it up and make the intention in the future that they are not going to return to it.

These are the basic conditions around repentance that have to be fulfilled for that du'a, sayyidul istighfar, to have a full impact. The impact which prophet Muhammad (ﷺ) said,

"If a person makes this du'a sincerelly in the morning and dies in the evening, or says it in the evening, and dies the next morning, their place will be in paradise." [1]

That is the promise of the Prophet (ﷺ).

Reference:

[1] [Sahih Al-Bukhari:  Book 20, Hadith 1875]