The Math Expression node computes a complex math expression defined through the Expression property (for more details please refer to this page). After the expression has been defined, the variables contained in the expression will become the input connectors of the node.
For each Input connector, the Trigger property determines whether a new value received on the connector triggers the computing of the expression (by default this property is enabled).

The Trigger property must be disabled when the input connector and the output connector are linked to the same source (for example the same KNX group address) to avoid infinite loops.