The feature existis, as said, only once the flight is already within the check-in window. Of course, the seat next to you has to be empty when the flights goes into CKI, otherwise the seat blocking can't work.
Take LH778 FRA-SIN tomorrow: Seat 81A is occupied, and seat 81C is blocked (as indicated by the "-" designator on the seat map).
Code:
FLT IN CKI
SM LH 0778 F 22JAN FRASIN 744
A C H K
F 81 UP + - + . UP 81 F
82 + + . + 82
83 E + + . . E 83
84 UP . . . . UP 84
A C H K
Here are a few more examples:
Code:
FLT IN CKI
SM LH 0772 F 22JAN FRABKK 744
A C H K
F 81 UP + + - + UP 81 F
82 + + . . 82
83 E . + - + E 83
84 UP . . + - UP 84
A C H K
Code:
FLT IN CKI
SM LH 0762 F 22JAN MUCDEL 346
A D G K
F 1 + . + + 1 F
2 + + - + 2
A D G K
Code:
SM LH 0506 F 22JAN FRAGRU 744
A C H K
F 81 UP + + + + UP 81 F
82 + + + + 82
83 E - + . . E 83
84 UP + - - + UP 84
A C H K