Mana Short — ruling 1

Since this is an instant, your opponent may use instants in response to the casting of this spell. Note that such spells and abilities will be resolved before mana is removed by Mana Short. This makes it useless as a countermeasure for spells. To use it effectively, you need to use it during Upkeep and even then it will not prevent the use of instant spells and abilities.

Mana Short — ruling 2

If you play Mana Short in response to a spell, it will have no effect on that spell since the mana has already been paid.

Mana Short — ruling 3

It even taps lands that do not produce mana.

Mana Short : règles, fonctionnement et interactions (MTG)

Mana Short is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Tap all lands target player controls and that player loses all unspent mana.

How it works : Mana Short

Original English text — Wizards of the Coast / Scryfall

#1October 4, 2004· Wizards of the Coast
Since this is an instant, your opponent may use instants in response to the casting of this spell. Note that such spells and abilities will be resolved before mana is removed by Mana Short. This makes it useless as a countermeasure for spells. To use it effectively, you need to use it during Upkeep and even then it will not prevent the use of instant spells and abilities.
No other card is named in this ruling.
#2October 4, 2004· Wizards of the Coast
If you play Mana Short in response to a spell, it will have no effect on that spell since the mana has already been paid.
No other card is named in this ruling.
#3October 4, 2004· Wizards of the Coast
It even taps lands that do not produce mana.
No other card is named in this ruling.