When a meditator stops generating all kamma saṅkhāras, (that is, new actions or reactions), the old impurities—pure kataṃ rajaṃ—are combed out. Dhunamānassa means combing or carding cotton, separating every fibre, clearing out all the knots and dirt. This can happen at any stage, whenever you don’t generate a new saṅkhāra, but the very deep-rooted impurities only start coming up after bhaṅga. If you keep generating saṅkhāras, you keep multiplying your old stock. As long as you refrain from generating any new ones and remain equanimous, layers after layers of saṅkhāras are eradicated.