Trade health for increased power.
You subtract 10 health from your health and add it to your next attack roll with weapon, spells or modules.
If you have lowered the health subtraction, this also affects any enhancement. Thus, if you have spendt 1 AP to lower the subtration from 10 to 8 and then go for the +10 health->attack roll enhancement with a +20 to Difficulty rating, then you only subtract 16 health but still add 20 to your attack roll.