A stiffer front ARB would increase the understeer tendency. A stiffer rear ARB would make it less understeery/ introduce oversteer.
I found that strange when I first learned about it
Me too, seems counter-intuitive, but it's correct. It's about the front vs rear distribution of the total lateral weight transfer, and how the relative stiffnesses of the front vs rear ARBs and springs affect this.
To understand this, it's also necessary to understand that while a tyres' grip increases with increased weight (vertical load), its' grip doesn't increase in a linear manner. Because of this, two equally loaded tyres will cumulatively generate more grip than the same two tyres if they are unequally loaded. This is hugely important to understand to get a handle on how cars (are likely to) respond to changes in spring and ARB rates.
The greater the 'roll stiffness' the more 'weight' will laterally transfer from the inside to the outside wheels, and the axle line (i.e. either the front wheel pair or rear wheel pair) with more roll stiffness will transfer more 'weight' from the inside tyre to the outside tyre than the axle line with relatively less roll stiffness. Less weight transfer at an axle line means that both wheels on that axle line are sharing the vertical load more equally than if the weight transfer is greater, which means effectively more 'rubber on the road' (albeit a simplistic concept) at that axle line, so more grip at that axle line (than at the other axle line with relatively more roll stiffness).
Roll stiffness comes from the springs and from the ARBs (and also the suspension geometry, and transiently the damper stiffness, but lets ignore that, to avoid pages and pages of discussion...). Stiffer springs = greater roll stiffness, stiffer ARB = greater roll stiffness. If the ARB were very weak or made from wet spaghetti, how much load could it transfer from the inside wheel to the outside wheel? If it were very thick or made from 'unbendimum', how much load could it transfer?
So, if the front roll stiffness is higher than the rear roll stiffness (because of relatively stiffer front springs, or ARB, or both, it's the total that matters), there will be more lateral weight transfer at the front of the car, from inside front tyre to outside front tyre. This is why increasing the front ARB stiffness will will create an understeering tendency because front grip will be less relative to rear grip, because less lateral rear weight transfer occurs (because there is less roll rear roll stiffness). If the rear roll stiffness were to be relatively higher than the front roll stiffness, then there will be an oversteering tendency, being why fitting a stiffer rear ARB decreases understeer (i.e. 'adds' some degree of oversteer).
'All else being equal' applies here, such as static front to rear weight distribution, suspension geometry, tyres and pressures, etc. etc. etc.
Regards,
John.