Tried multiple times and it finally went through
You just add the strength to one chosen character that isnt the character doing the action
This can produce some pretty serious blowouts when your creature wins combat
The great thing about Jumpstart is that its cheap a new account grants you enough currency to try the mode three times and the cards you pick are yours to keep

$2.29 Kain, Traitorous Dragoon (0316 - Borderless) Final Fantasy Variants $0.59 Memories Returning (0331 - Borderless) Final Fantasy Variants $1.99 Jecht, Reluctant Guardian (0363 - Borderless) Final Fantasy Variants $3.49 Gilgamesh, Master-at-Arms (0338 - Borderless) Final Fantasy Variants $1.99 Excalibur II (0352 - Borderless) Final Fantasy Variants $2.99 Ultima, Origin of Oblivion (0324 - Borderless) Final Fantasy Variants $1.99 Stiltzkin, Moogle Merchant (0327 - Borderless) Final Fantasy Variants $1.49 Sin, Spira's Punishment (0348 - Borderless) Final Fantasy Variants $5.49 Venat, Heart of Hydaelyn (0329 - Borderless) Final Fantasy Variants $2.99 Moogles' Valor (0326 - Borderless) Final Fantasy Variants $2.49 Zodiark, Umbral God (0336 - Borderless) Final Fantasy Variants $7.99 Summon: Primal Odin (0365 - Borderless) Final Fantasy Variants 0 results found
Promo cards follow their own sequence, typically starting with P (e.g., P1, P2) or a code related to a specific event