Unlike most normal attacks, which leave the opponent standing or crouching after hit, attacks that cause knockdown will knock down the opponent as the name implies. Afterward, the opponent will be left in a lying down state. In most fighting games, two types of knockdowns exist: soft knockdowns and hard knockdowns.
A soft knockdown will cause the opponent to go into a lying down state on hit. Once the opponent touches the ground, they will be able to tech the knockdown, causing them to get back up into a standing state much sooner than they normally would. In some games, characters can also perform a recovery roll after a soft knockdown. Additionally, a character affected by soft knockdown usually cannot be hit by an OTG due to the short window of time and/or the ability to recover quickly. This type of Knockdown is usually performed with Special moves.
Due to the way the default states in M.U.G.E.N are coded, most attacks that are coded to perform Knockdowns cause Soft Knockdowns by default.
A hard knockdown (also known as an untechable knockdown) behaves similar to a soft knockdown, in that it will cause the opponent to go into a lying down state on hit. The main difference, however, is that the opponent will not be able to tech the knockdown or perform a recovery roll, and will be forced to stay in a liedown state for some time. During this time, the opponent will be completely vulnerable to OTG attacks. These types of knockdowns are performed almost exclusively by throws, command grabs, sweep kicks and Hypers.
Due to a bug in the M.U.G.E.N engine that handles the transition from a liedown state to the getup state from a knockdown, hard knockdowns are not possible in M.U.G.E.N without the use of custom states or overwriting the common1.cns file in M.U.G.E.N's data folder.