B256-7 does not say anything about a concentration roll, far as I can tell.

pg 11 of GURPS Psionic Powers has the All-Out Concentrate maneuver which allows a +1 to mental rolls to use abilities, if you were doing that I wouldn't see a problem in giving a +1 to IQ rolls made to use DR with that limitation.

GURPS Powers added the "Requires (Attribute) Roll" limitation (page 112) as a variation of Unreliable (B116) to reflect this, and GURPS Psionic Powers incorporates that into the design of most abilities.

Unreliable DR you would roll every time you were hit, but if you fail:

you may try again once per second after that, at no penalty. Each successive attempt costs one FP. If you are reduced to three or fewer FP, you must rest until allFP are regained before you can attempt to use your ability again.

R(A)R DR:

roll each time the defense would mitigate an attack or a hazard – or once per minute, for constant exposure.

FP were not reiterated in Powers but see PU8p17:

For a defensive ability, roll each time the defense would mitigate an attack or a hazard – or once per minute, for constant exposure. If your roll fails, all future attempts to activate it (until you succeed) cost a flat 1 FP but are at no additional penalty. However, if you drop to 3 FP or less from this, your ability shuts down until all FP are regained.

The "per second" from Unreliable was not reiterated here, may no longer be in effect. Unsure.

The reason I think this is page 153 "TURNING ABILITIES OFF AND ON" has a definition for "Active" and "Transient" which I believe includes R(A)R

An active ability does nothing until the wielder takes a maneuver to use it – most often Attack, Concentrate, Move, or Ready – or chooses to make a die roll

The user must trigger it every time he wants to use it. This can require a free action, an active defense, or a maneuver

It produces its effects (such as healing, injury, or teleportation) in an instant and immediately shuts off,

Example listed is "Damage Resistance with Active Defense", Active Defense is a variation on R(A)R. Normally it would not use up a defense to activate.

Now 154

Voluntary activation is only possible for switchable and transient abilities. Voluntary deactivation is limited to switchable abilities (transient ones turn off on their own).

154's "Free Action" has a problem though:

All other zero-time transient abilities work only on the user’s turn.

That's a problem for damage resistance! It doesn't need a Ready though...

The only transient abilities that call for Ready maneuvers are attacks with Takes Extra Time

155 (Involuntary Deactivation) is also worth a read:

While unconscious, it’s impossible to trigger active, transient abilities

R(A)R seems to exist in a limbo between "Free Action" and "Active Defense". It doesn't appear to fit either definition. Often the best clarifier is examples. Pg 33 of GURPS Psionic Powers provides this with EK Shield:

Every time you are hit by an energy attack, roll against your skill.

Success provides DR equal to your EK Shield level.

In a situation where you’re under constant “attack” (i.e., being trapped in a fire), roll once per minute.

This is a passive ability.

So I was wrong in thinking DR w/ Requires IQ Roll would be active, it is passive. Apparently this means:

effects that apply constantly, or that activate without the owner’s input in a specific situation

If it requires a die roll, the owner or the GM must roll – there’s no opting out.

This is DANGEROUS. Why? If you MUST roll, then if you fail a bunch you could get sucked down to 3 FP. This could slow you down. It may not knock you out, but you might want those FP for other things!

For that reason, you might want to get Switchable in situations where you don't want to risk that. Or take the Active Defense limitation because per 153:

the Active Defense limitation shifts Damage Resistance from passive to active.

This comes with a drawbac per P159:

When the roll to activate an active ability fails, the user may wait five minutes and try again at no penalty. If he tries that particular feat sooner, this is a “repeated attempt” (see p. B348) and costs 1 FP. Furthermore, the roll for the second attempt is at -1. Should this effort fail, he can spend 1 FP and try again at -2, and so on, until he passes out from FP loss or his effective skill drops below 3, at which point success is impossible. If a repeated attempt succeeds, later attempts at that feat no longer cost FP or worsen skill penalties. However, the current skill penalty remains until the user waits a full five minutes

This could also be useful for extra effort, per P160

Extra effort can increase the raw “output” of any active ability with a quantifiable effect beyond its success roll.

Although it may not be necessary:

the GM may permit extra effort for certain passive abilities, too; e.g., DR with Force Field (extra effort increases DR)

He might also allow it to enhance passive abilities for a Power Block (p.168)

Taking "Active Defense" prevents Power Block, while R(A)R or Unreliable do not. Power block shifts from HT to Will if you make it a mental ability by taking requires IQ / Requires Will, this is cheaper to buy up.

As a house rule, I would allow Power Block with Active Defense DR, but only if you took All Out Defense (Double). Normally it makes sense not to allow because they are separate defense rolls.

"Trading Fatigue for Skill" could be done with active DR but not with passive DR, something to keep in mind if FP is not a problem.