How each monthly payment splits between interest and principal. Early on, most of what you pay is interest; by the end, almost all of it goes to principal. That's why paying extra in the first few months saves far more interest than paying extra near the end.
Two amortization styles show up in Mexico. French style (the standard on personal loans and mortgages here): monthly payment stays flat, interest is front-loaded so early payments barely dent principal. German style (rare, mainly commercial): principal is flat, monthly payment declines over time. On a 100,000-peso loan over 24 months at 30% CAT, the French monthly is about 5,650 pesos and total interest lands near 35,600. On a 12-month term the same amount is roughly 9,750 monthly with 17,000 in interest.
Prepayment always applies to principal by federal law, which cuts future interest proportionally. That means paying an extra 5,000 pesos in month 3 saves noticeably more than the same 5,000 paid in month 20 — the front-loaded interest schedule works against you if you sit tight.