When you attack a player, a percentage in between the aforementioned parameter is added to their damage. When it reaches the second percentage amount, there is a 50% chance that character will die. Every 5% over that amount, the chance goes up another 10%, capping at 99%. Let's imagine it in action:
Character A does 10-15% damage every turn
Character B has 100% defense
Character A eventually gets Character B to 100% damage. At this point, there is a 50% chance Character B dies. Let's say Character B survives, and the next day Character A does 15% to Character B. Now, Character B has 115% damage. Since this is 15% over, that adds an additional 30% chance that Character B will die, so there is an 80% chance Character B will die.