The affix ṇamul is added to the root when a noun in the Ablative -5th case is in composition and when 'haste' is intended.,
The word परीप्सा means 'haste, hurry'. Thus शय्योत्थायं (or शय्याया उत्थायं) धावति 'having risen from bed, he runs away'. That is to say, he runs with such a haste that he does not perform even the necessary ablutions &c, but as soon as he rises from the bed, he runs off. Similarly रन्ध्रापकर्षम् पयः पिबति 'he drinks milk from a hole in the vessel', he being in such a haste, that he does not care to drink from the proper aperture. भ्राष्ट्रापकर्षमपूपान् भक्षयति 'he eats cakes hot from the frying pan', not waiting till they are placed on a dish.
Why do we say 'when meaning hurry'? Observe, आसनादुत्थाय गच्छति 'having risen from the seat, he goes'. Here 'haste' not being intended, the affix क्त्वा (ल्यप्) is employed. See 2.2.21.,