Send x2many as parametr to onchange metod. It will have list of records as following: [[Mode, Option, {'field_name':field_value_record1, ...}], [Mode, Option, {'field_name':field_value_record2, ...}]]

Combination of Mode, Options and Fields Values define state of x2many records.

(0, 0, { values }) link to a new record that needs to be created with the given values dictionary

(1, ID, { values }) update the linked record with id = ID (write values on it)

(2, ID) remove and delete the linked record with id = ID (calls unlink on ID, that will delete the object completely, and the link to it as well)

(3, ID)
cut the link to the linked record with id = ID (delete the relationship between the two objects but does not delete the target object itself)

(4, ID) link to existing record with id = ID (adds a relationship)

(5)
unlink all (like using (3,ID) for all linked records)

(6, 0, [IDs])
replace the list of linked IDs (like using (5) then (4,ID) for each ID in the list of IDs)

For example: [4,5,False] - define existing records in DB with id=5

[0,0,{'name':'new record'} - difine new record with field "name"="new record", that will be created in DB, when holder of x2many will be saved to DB.