Table1 structure:

Recno    Field1        Field2
------    ---------    --------
1            12345
2            12367
3            92341
4            12312
5            <empty>
6            56789
7            <empty>

update table1 set field2=substringfield11 from 4 for 2)

Recno    Field1        Field2
------    ---------    --------
1            12345        45
2            12367        67
3            92341        41
4            12312        12
5            <empty>    12   <----------------- I think it must be empty 
because substring is ''
6            56789        89
7            <empty>    89   <----------------- I think it must be empty 
because substring is ''