Hello!
Did I understand correctly the UNDERCALL with no verbal declaration:
1) Check + Open bet + Fold + Call + Undercall > THE UNDERCALL MUST BE COMPLETED
(because here we only face the open bet, even after the first call that changes nothing right?)
2) Check + Open bet + Fold + Raise + Undercall > THE UNDERCALL CAN BE COMPLETED OR FORFEIT
(because here we don't face only the open bet but multiple bets right?)
3) Big Blind + Fold + Raise + Undercall > THE UNDERCALL CAN BE COMPLETED OR FORFEIT
(because the Big Blind is actually an Open bet right?)
Conclusion (if I'm right) : The most numerous cases oblige to complete the Undercall.
Finally only the raises can give the choice to forfeit the Undercall.
Tell me ... :)