VAL('dataname')
IF(ISBLANK({{oneField}}), VAL('otherField'), {{oneField}})
This formula will evaluate once when the form loads and thereafter will only ever re-evaluate when the value of 'oneField' changes. This formula is not dynamically dependent on 'otherField' in any way since it does not contain a dynamic reference {{otherField}}. When the value of 'otherField' changes, this formula will not re-evaluate.
It is not dynamic when used alone, returning the value of the field referenced, 'dataname'