You would add a new column to the cart object to hold the category. For all Drills, set the value of this column to "drill". if you are going to have different discount amounts for each category, you could also add a new column to hold that value. for each item in the drills category, you would set the discountAmount column to "0.01"
Then create a calculation to calculate the discounted price for drils and the regular price for all others:
truePrice:
(abs([catagory] == 'drills') * ([Price] - ([Price]*[discountAmount]))) + (abs([catagory] != 'drills') * [Price])
and modify the Total Price calculation:
TotalPrice:
((abs([catagory] == 'drills') * ([Price] - ([Price]*[discountAmount]))) + (abs([catagory] != 'drills') * [Price])) * [Quantity]
You can find more information on manipulating prices through calculations here:
ecart_rules.pdf