Label: original alphabet rule. Hand shifts. No program was run.

Alphabet, A=0 through Z=25:
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Encrypt: add the shift, then wrap with modulo 26.
Decrypt: subtract the shift, then wrap with modulo 26.
The stated shift on these cards is +1.
A steps back to Z. Z steps forward to A.

Worked encrypt: HI -> IJ.
H=7 becomes 8=I. I=8 becomes 9=J.
Worked decrypt: IJ -> HI.

Activity decrypt, shift +1: ABU.
A steps back to Z. B steps back to A. U steps back to T.
Predicted plaintext: ZAT.