General Mechanics

There are several hidden mechanics of 13.5

Meter
Meter is measured in units. It takes 25 units to gain one meter; one hit results in 2 units for the attacker. The receiver of the attack gains meter depending on the round number. For example, in the 5th round, it takes 5 hits from the attacker to result in one meter.

All attacks result in the same amount of meter gain as any other attack. However, some certain attacks do not result in meter, like Akasaka's 46C. Most notably, most Last Word/Mysteries do not result in any meter, as the resulting meter gain would be ridiculous with 46 hit attacks.

Another noteworthy attack would be Mystia's j.236236A: it is the only attack in the game (?) that results in more meter than is used to execute it.

Proration/Damage Reduction
Damage reduction is a linear function, calculated as Percent of health gone/2 %. That is, if the opponent is at 100% HP, there is 0% damage reduction, while at 0%, there is 50% damage reduction.

There is no combo proration; in other words, every hit of a combo does however much damage it is supposed to do at that given health value.

Chip Damage
Chip damage is affected by Proration/Damage Reduction, and is usually applied at a 20% rate, though certain attacks have no chip damage. Chip Damage only appears in the newer versions.

Groundslam and Wallbounce
Groundslam and wallbounce can only be done once each per combo. For example, if Lambdadelta and Bernkastel chain a 236236B (groundslam) into a 236B (wallslam), another 236236B will not cause groundslam. Likewise, chaining a 236B into another 236B does not cause wallslam twice.