What is the concept of Sadd al-Dhara'i' (blocking the means), and what is the comparison between it and legal stratagems (Hiyal)?

Blocking the means (Sadd al-Dhara'i') is the prohibition of something permissible or apparently permissible if it leads to something unlawful. Imam Malik and Imam Ahmad adopted this principle, citing as evidence the verse: "And do not insult those they invoke other than Allah, lest they insult Allah in enmity without knowledge" (Quran 6:108), and the Prophet's (peace be upon him) prohibition of a man cursing his parents by cursing the parents of another. The Ummah agrees on blocking the means if it leads to a certain evil. A condition for this is that there should be no expansion in blocking them to avoid hardship and difficulty. Scholars also permit opening the means (Fath al-Dhara'i') to commit what is unlawful if refraining from it leads to a greater harm. The conditions for this are that the harm cannot be prevented by a permissible act, and that the harm is greater than the unlawful act committed. Summarizing this is the saying of Shaykh Sidi Abdullah al-Shinqiti: "Blocking the means to the forbidden is a must, just as opening them to what is necessary."