You can fight but the amount of damage you do according to you is not what the system detects, that is when you are below the pb, if you are above you can put the multiplier by 100 , and when your bp is below there is a certain limit to use the damage multiplier, for example let's say you can be 1k below and still be able to use the multiplier then the difference in bp will now be 2k you try to use the damage multiplier and it detects it,It is more related to the damage you are doing by having a negative bp difference to the instance