Triggers are recorded in database as record IDs (together with the model name) and refer to the workflow instance waiting for those records. The transition definition provides a model name (attribute trigger_model) and a Python expression (attribute trigger_expression) that evaluates to a list of record IDs in the given model. Any of those records can wake up the workflow instance they are associated with.
Creating Triggers
The syntax for creating a trigger is:

CREATE [OR REPLACE ] TRIGGER trigger_name 
{BEFORE | AFTER | INSTEAD OF } 
{INSERT [OR] | UPDATE [OR] | DELETE} 
[OF col_name] 
ON table_name 
[REFERENCING OLD AS o NEW AS n] 
[FOR EACH ROW] 
WHEN (condition)  
DECLARE
   Declaration-statements
BEGIN 
   Executable-statements
EXCEPTION
   Exception-handling-statements
END;

Note-triggers are not re-installed whenever the transition is re-tried.