How does Korean distinguish aspirated consonants from tense consonants?

The Korean language distinguishes between two specific types of obstruents—heavily aspirated consonants and reinforced (tense) consonants—which differ from the voicing distinctions found in English.1

• Heavily Aspirated Consonants: These include p’, t’, k’, and ch’. They are characterized by a strong burst of breath during articulation. Historically, these sounds are believed to have evolved from earlier clusters containing h or k that were reduced through the process of syncope.1
• Reinforced (Tense) Consonants: These include pp, tt, kk, and tch. They are written as geminates (duplicate letters) in Hangul. These sounds likely emerged from the reduction of Middle Korean consonant clusters such as st, pst, and pt.1

Additionally, reinforcement can occur automatically in certain phonetic environments. For example, when p, t, k, ch, or s follows a stop, it is pronounced as a tense consonant, though this feature is often omitted in standard spelling and transcription.2