The best description of tadabbur that I've come across is one by Sheikh ʿAbdur-Raḥmān as-Saʿdī. He was asked, “How do I reach a point where I can do tadabbur of the Qurʾān (reflect on the Qurʾān)?” He simply said, ❝You continue to read and read and read*... until Allāh opens your heart to tadabbur.❞ I haven't found a truer description than this or description closest to how the pious predecessors understood tadabbur.
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Ibn Hubayrah رحمه الله:
“From the devil's trick is to turn people away from Tadabbur (pondering on) Qur’ān for he knows guidance comes from Tadabbur.” [ذيل طبقات الحنابلة ٢/١٥٦]