Heightened Difficulties (POSP)

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An additional benefit of this system is that the DM can set particularly difficult checks for characters with high ability scores—requiring two or more successes on the characters' multiple d20 rolls.
For example, if the iron chest Leon's fighter wants to lift had been welded in place, the DM could claim that two or even all three of the fighter's d20 rolls be successful for the chest to come free. This further reinforces the idea that characters with low scores are not as capable as their comrades with higher scores.
For those occasions when a lone adventurer cannot succeed at an ability check due to insufficient dice, additional characters might be able to help. If the DM rules that more than one character can attempt the feat, roll the d20s of the character with the highest ability score, then add 1d20 for each additional character assisting the primary character.
This system also can be used to determine the outcome of direct ability-vs.-ability contests between characters. The winner of such a contest is the one who successfully rolls the highest result—if that result is equal to or under the ability score in question. For example, if two characters with 14 Aim scores compete to see who can hit a far-off boulder with thrown stones—and the results of the d20 rolls are a 14 and a 6—the character who rolled the 14 wins the contest and strikes the boulder first. Both characters' rolls succeeded, but the 14 is a higher degree of success than the 6. If the character with the 14 result had instead rolled a 15 or a 5, the other competitor would have won, in the first case because 15 is not a successful Aim check, and in the second because a 6 is higher than a 5.
Another common example of such a contest is an arm-wrestling match. Marshall has a ranger with a Strength of 15 and subability scores of Stamina 13 and Muscle 17. Brian has a fighter with a Strength of 13 and subability scores of 13 in both Stamina and Muscle. These characters decide to arm wrestle to see who gets the first choice of treasure. The first ability check would be against Muscle to see if either character could overwhelm the other with a burst of Strength. Marshall and Brian roll 1d20, and both Muscle check results are 11. The characters have matched power for power.
The next check (and all subsequent checks) would be against Stamina to see which character tires first. Marshall rolls a 7 and Brian rolls a 12. Both are successes, but Brian's roll was higher. Brian's character wins the arm-wrestling match despite having lower Strength and Muscle scores. Simply put, Brian's character got lucky, and the next match between the characters could turn out differently.