Hams literally suggests a subdued sound. As a technical quality, it describes the continuation of breath during the articulation of a consonant. It does not mean that the entire recitation must be whispered.
Its ten letters are gathered in the mnemonic فَحَثَّهُ شَخْصٌ سَكَتَ: ف، ح، ث، ه، ش، خ، ص، س، ك، ت.
Practice: pronounce a final unvowelled س or ف, as in «أَسْ» or «أَفْ», and notice the escaping breath. Compare it with unvowelled د in «أَدْ». The purpose is not to force out excessive air, but to preserve the letter's inherent breath-flow quality.