Wildborn Preserver — ruling 1

If you pay {X} while Wildborn Preserver's triggered ability is resolving, the reflexive triggered ability triggers and will resolve separately. This is different from effects that say “If you do . . .” in that players may take actions after you've paid mana but before the creature gets +1/+1 counters.

Wildborn Preserver : règles, fonctionnement et interactions (MTG)

Wildborn Preserver 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

Flash (You may cast this spell any time you could cast an instant.) Reach (This creature can block creatures with flying.) Whenever another non-Human creature you control enters, you may pay . When you do, put X +1/+1 counters on this creature.

How it works : Wildborn Preserver

Original English text — Wizards of the Coast / Scryfall

#1October 4, 2019· Wizards of the Coast
If you pay while Wildborn Preserver's triggered ability is resolving, the reflexive triggered ability triggers and will resolve separately. This is different from effects that say “If you do . . .” in that players may take actions after you've paid mana but before the creature gets +1/+1 counters.
No other card is named in this ruling.