HI

BU has SPR cca. 1.6
According to SPR, for BU it is mandatory stack off. He can not never fold this hand.
Why? What if I know that has only 88 and 55? Why it is mandatory stack off?

Scenario:
Effective stacks: 100bb
BU has A-A
BU opens, SB 3bets, BU 4bets, SB calls
FLOP: 852s (48bb)

Thank you so much!