Another player said that if you say "Pestilence for 6," it's assumed you mean "I'm planning to activate Pestilence's ability, let it resolve, then activate it again, repeating this for a total of 6 times." And if you're opponent wants to break the sequence with something like Prismatic Strands, they have to say when they're doing so.
In such situations where it could matter, I advocate walking through the Pestilence activations one by one. I feel like if someone said "I Pestilence for 6" and tap all 6 of their lands, it'd be assumed that they hold priority and put all 6 activations on the stack, that could then be stopped by a single Prismatic Strands in response. I feel like this is an area where you should be as clear as possible what you're doing with the stack. What do you think is the most common interpretation of "Pestilence for X?"
It is a tournament shortcut to assume, that each activation resolves before the next instance is put on the stack. So in a tournament setting you'd have to specify if you wanted to do all activations at once without letting any resolve.
Of course, it can't hurt to be explicit about the timing even if it is following the default and wouldn't need mentioning.
In a sanctioned tournament in general, a player that "adds a group of objects", such as multiples of Pestilence's last ability, "to the stack without explicitly retaining priority" adds them one at a time, "allowing each to resolve before adding the next" (M.T.R. 4.2, fourth bulleted item). Thus, in such a tournament in general, a player that says "Pestilence for 6" intends to activate Pestilence's last ability, to wait until it leaves the stack and they get priority again, to activate that ability again, and so on until that ability is activated six times this way. (This is given as a shortcut under the Magic: The Gathering Tournament Rules because technically, a player who had priority before activating an ability gets priority after doing so [C.R. 116.3c; M.T.R. 4.2; see also C.R. 720.1].)
So, what you should always remember is that players use shortcuts at their own risks. If they then create confusion or a situation of miscommunication, they are responsible for it.
Ex 1: Player A taps 3 swamps and says “pump”, pointing out at his Looming Shade. In response, player B plays Incinerate on the Shade. Even if A claims that he wanted to pass priority between each pump, he chose to use a shortcut (not making clear that he was passing priority), and then cannot come back on it. According to the second criteria of the intent test explained in Justus’ article, it is obvious that you can’t let A’s creature survive since, because of his shortcut (“sloppy play”), he now knows that B is holding an Incinerate in hand.
So, what you should always remember is that players use shortcuts at their own risks. If they then create confusion or a situation of miscommunication, they are responsible for it.
Ex 1: Player A taps 3 swamps and says “pump”, pointing out at his Looming Shade. In response, player B plays Incinerate on the Shade. Even if A claims that he wanted to pass priority between each pump, he chose to use a shortcut (not making clear that he was passing priority), and then cannot come back on it. According to the second criteria of the intent test explained in Justus’ article, it is obvious that you can’t let A’s creature survive since, because of his shortcut (“sloppy play”), he now knows that B is holding an Incinerate in hand.
Keep in mind that this article is fully 15 years old, and the MTR is super clear about how this shortcut works, "intent" or not:
Quote from MTR 4.2 »
If a player adds a group of objects to the stack without explicitly retaining priority, they are assumed to be adding them to the stack individually and allowing each to resolve before adding the next. If another player wishes to take an action at a point in the middle of this sequence, the actions should be reversed to that point.
This dates back to at least Jan 2011, which is the oldest MTR I have archived, using near-identical language.
In such situations where it could matter, I advocate walking through the Pestilence activations one by one. I feel like if someone said "I Pestilence for 6" and tap all 6 of their lands, it'd be assumed that they hold priority and put all 6 activations on the stack, that could then be stopped by a single Prismatic Strands in response. I feel like this is an area where you should be as clear as possible what you're doing with the stack. What do you think is the most common interpretation of "Pestilence for X?"
Corrupt Control B | Burn R | UG Turbofog UG | White Weenie W | GW Tethmos WG | BG Cycling Combo BG
Enchantress GBW | Colorless Tron C | Red Deck Wins R | UG Madness UG | Mono-G Tron G | UR Puzzlehorns UR
Rhystic Tron WU| WU Prowess WU | BR Reanimator BR | Mono-R Control R | Stompy G | Temur Tron URG
Mardu Infinite Priest WBR | 85-Card Dredge BRG | Elves GU | Boros Bully RW | Jeskai Familiars RWU
Of course, it can't hurt to be explicit about the timing even if it is following the default and wouldn't need mentioning.
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Corrupt Control B | Burn R | UG Turbofog UG | White Weenie W | GW Tethmos WG | BG Cycling Combo BG
Enchantress GBW | Colorless Tron C | Red Deck Wins R | UG Madness UG | Mono-G Tron G | UR Puzzlehorns UR
Rhystic Tron WU| WU Prowess WU | BR Reanimator BR | Mono-R Control R | Stompy G | Temur Tron URG
Mardu Infinite Priest WBR | 85-Card Dredge BRG | Elves GU | Boros Bully RW | Jeskai Familiars RWU
This dates back to at least Jan 2011, which is the oldest MTR I have archived, using near-identical language.