In addition to features of PostgreSQL, triggers can be created with DO option.

CREATE [ OR REPLACE ] [ CONSTRAINT ] TRIGGER name { BEFORE | AFTER | INSTEAD OF } { event [ OR ... ] }
    ON table_name
    [ FROM referenced_table_name ]
    [ NOT DEFERRABLE | [ DEFERRABLE ] [ INITIALLY IMMEDIATE | INITIALLY DEFERRED ] ]
    [ REFERENCING { { OLD | NEW } TABLE [ AS ] transition_relation_name } [ ... ] ]
    [ FOR [ EACH ] { ROW | STATEMENT } ]
    [ WHEN ( condition ) ]
    { EXECUTE { FUNCTION | PROCEDURE } function_name ( arguments )
        | DO [ LANGUAGE lang_name ] code }

Refer to the PostgreSQL Documentation for information about CREATE TRIGGER. This section describes DO option.

A trigger which is created with DO option will be associated with the specified table or view and will execute the specified code by the specified procedural language of DO (unnamed code block) when certain events occur.

lang_name

The name of the language that the function is implemented in.

plpgsql is supported in CREATE TRIGGER.

code

When the certain events occur, it executes the code in a specified procedural language. The unnamed code block does not require a prior definition like a function. Syntax is same as procedural language.

It executes the code block that is specified by DO before the table is updated.
(Example that LANGUAGE is plpgsql)

CREATE TRIGGER check_update
    BEFORE UPDATE ON accounts 
    FOR EACH ROW 
    DO $$BEGIN RETURN NEW; END;$$ ;