FS are an extension of the way combat works, an example
2 units of 6 str the odd's are 50/50
Each time a successful blow is landed, one the units takes x amount dmg, the dmg either unit recieves per successful blow is the same.
Lets say the differance is a 9 str vs a 6 str unit.
The dmg is now calculated differantly a successful strike on the 6 str unit will deal x amount of dmg, if a blow is landed on the 9 str unit it's dmg will be 2/3 of figure x, putting that into some rough ballpark numbers (not accurate, but close) the 6 str unit will deal somthing like 16/17 hp dmg per blow, the 9 str unit will deal 24/25 hp per blow.
In cases were your strength is double, the dmg you inflict is double that you receive so the other unit would need to score over twice as many hits on you to to win the round.
If you have a high str rating an several FS what that means is you will likely do dmg around the mid 30s (hp per hit) so 3 or 4 hits an your opponent is dead, meaning that if you have a strong unit with lots of FS you can fight battle after battle with no loss, or very low loss of health.