Trades reduced damage for a lower weapon speed factor.
You can treat the Speed Factor of this weapon as if it were 2 points lower than it’s value, but if you succesfully deal damage you reduce your weapon damage dice by 1 dice step.
Using a weapon with the Swift Feature has a special interaction with weapons who’s dice step would be lowered below a d4 (if damage dice could be). If your weapon damage dice could be reduced below a d4 when you use this feature, you treat results of 4 on the dice as if they were a 3 instead.