Suppose I'm playing MHRP (or a fantasy version of it), and my wizard hero has a series of power sets, with the overarching instruction All these power sets begin shut down, but at the start of each session, and during a Transition Scene, you may choose to restore one of them. After you use one of these power sets in an action, it shuts down at the end of that Action Scene.For the most part we agree on the above point, although I reiterate that predictability is important, not merely so the GM can avoid social opprobrium but so players can make informed decisions about issues their characters would understand without having to play Mother May I, which breaks flow.
Imagine playing a 5E wizard trying to decide between preparing Haste and Invisibility before infiltrating a hobgoblin citadel, but you don't know any of the rules of 5E so you are reduced to asking your GM for educated guesses about what the different spells might do in hypothetical scenarios, and his answer is usually "it depends." It's hard to meaningfully roleplay a wizard when you don't know the things a wizard should logically know!
Two of these power sets are Spell of Invisibility (Invisibility d10) and Haste Spell (Enhanced Reflexes d8, Enhanced Speed d8).
As a player, at the end of the Transition Scene where we are preparing to infiltrate the enemy citadel, I have to choose which power set to restore (in the fiction, which spell to memorise).
If I'm not sure how action declarations and action resolution work in MHRP, I can ask the GM or I can read the rules. But these won't tell me what my spells might do in hypothetical scenarios, beyond the rather obvious:
At d10, [Invisibility] renders you completely invisible to standard visual means. You can move around without being noticed, and you don’t leave a shadow or other signs of being there. Certain spectrums of visual detection may spot you, such as infrared or dimensional locators.
At Enhanced Speed d8, you can run at the speed of a horse
Enhanced Reflexes d8 represents two to three times the normal human response time and hand-eye coordination.
At Enhanced Speed d8, you can run at the speed of a horse
Enhanced Reflexes d8 represents two to three times the normal human response time and hand-eye coordination.
What I can actually do with those effects is up to my imagination (and the luck of the dice). The GM has nothing especially useful to tell me about it.
The risk of "Mother May I" and the role of rules in obviating that particular risk is a function, I think, of particular approaches to defining players' abilities within the rules.