Hello! Combat guy here - let me try to shed some light on the design here:
Power % - Power scales the base of any attack, it is then used to determine amount of Damage (straight up hitpoints), amount of Impact (to stagger enemies) and Cleave.
Rending: Your attack will disregard Flak or Carapace armor mod and use base damage instead for the rending portion. So at 50% rending, half the damage will calculate based on original armor type.
Brittleness: Rending but for all future attacks vs that enemy - basically provide rending for friends.
Chain: Attacks have timed chain-windows so basically if you continously keep attacking, chaining from one attack to another, you count as chaining. Chains start on the second attack in a row so any gains should be applied after that.
Impact: The stagger-potential of the attack. Each attack has an Impact number, it is checked vs the stagger thresholds of the target and then picks a type of stagger (light-medium-heavy) and scales duration/distance based on how far you exceed the threshold. More Impact equals more/longer control.
Basically what Padds said 
- Hope this helps, we’ll be tuning and tweaking stuff in the future so any feedback is appreciated.