I use this concept myself, and with clients.
I think it describes very well what happens when we hold onto anger beyond the point at which it is useful to us – and when we hold a grudge.
In the first flush we often want to let someone else know how we feel.
We might want action, change, compensation, an apology or restitution for something specific – and we may feel, or be, justified.
Or we might be angry at ourselves, or spurred on towards some other action.
But if we hold on to our anger after that, especially long term, I’ve found that it just eats away at our insides, just like the poison Buddha mentions.
Don’t get me wrong, I think anger has its place in the world. There are lots of good things, good changes, that have come about because people got angry and took action to highlight and address injustice, cruelty, or negligence, for example.
And how it’s expressed bears consideration, too. Aristotle had the right idea, I think, when he said:
“Anybody can become angry – that is easy,
but to be angry with the right person
and to the right degree
and at the right time
and for the right purpose,
and in the right way –
that is not within everybody’s power
and is not easy.”
This is a checklist we can use to help ensure that what we do as a result of feeling angry is well-targeted, appropriate and most likely to be effective.
After that, I recommend letting it go… 
Whole People 