Example:
You use axe wich makes 100-120 damage.
You need 50 Str.
If you have 40 str: (-20%)
You make 80-96 damage.
If you have 25 str: (-50%)
You make 50-60 damage.
if you have 5 str: (-100%)
You make 0-0 damage.
It means, your not able to make any damage with this weapon (you cant move it

)
The weapon bracke faster.
40 str 20%*5 .. the weapon bracke 100% faster
25 str. 50%*5 250% faster.
So if someone give a newbie huge weapon, he will be not able to make so much damage, and will not use it a long time.