Bhagavad Gita Verse 11, Chapter 5

kaayena manasaa buddhyaa kevalairindriyairapi |
yoginaha karma kurvanti sangam tyakvaatmashuddhaye || 11 ||
 
Solely with body, mind, intellect and senses does the seeker perform actions, casting off all attachment, for the purification of the self.
 
kaayena : with body
manasaa : mind
buddhyaa : intellect
kevalaih : solely (without a sense of mine)
indriyaihi : senses
api : only
yoginaha : seeker
karma : actions
kurvanti : perform
sangam : attachment
tyakvaa : cast off
aatmashuddhaye : purification of self
 
In the previous shloka, Shri Krishna explained how a karma yogi offers his actions to Ishvara. But how exactly does he do that? Shri Krishna in this shloka says that the karma yogi continuously keeps the thought that the body, mind, intellect and senses - all these are the property of Ishvara, given by Ishvara. Therefore, all the work done is for the benefit for Ishvara, and not for the ego.
 
The Gita teaches that the body, mind, intellect and senses are not really "ours". But due to our conditioning, we come to think that they are ours. We then begin to do things for their benefit. For example, the tongue likes sweet food, so we undertake actions to satisfy it by eating things that may not be healthy from a long term perspective. When actions are done for the satisfaction of body, mind, intellect and senses, the sense of "mine-ness" develops and the ego gets strengthened. We then begin to serve the ego.
 
In this regard, let us recall example of the CFO we had encountered a while ago. As long as the CFO thinks that she is working for company, things will go smoothly. But if she begins to think that working for herself, starts misusing the company's funds as her own funds, she gets into trouble. Similarly, a karma yogi treats his body, mind, intellect and senses as instruments of Ishvara. When that attitude comes, all actions and their results also get offered to Ishvara. In this manner, the seeker detaches himself from the body, mind, intellect and senses from the ego,  and attaches them to Ishvara. This is what is meant by "casting off all attachment".
 
Now, let's examine the second part of shloka where Shri Krishna talks about purification of self. What happens when karma yogi continues to dedicate actions to Ishvara? His mind gets cleared of all selfish desire, and his ego begins to lose its power. The end result is that the mind is purified of all vaasanaas - which is what is meant by "self" in the shloka. Purification of mind brings the karmayogi closer to self-realization.