What is the explanation of the hadith in which the Prophet, peace and blessings be upon him, commanded the young man to look at his fiancée, and does "If you are able to look at that which will encourage you to marry her, then do so" permit looking at what is not normally permissible?

The Hadith in question is authentic, and it contains an instruction to look at the woman one intends to marry, because it is more conducive to the permanence of affection between the spouses. As for after marriage, looking reverts to the general ruling on looking at a non-mahram woman.