Comment by Aurornis

This is completely false.

The ECU turns on the secondary air system and enriches the fuel mixture so the exhaust temperature goes up, heating the catalytic converter rapidly. Catalytic converters must be hot to work, so getting them hot quickly is important.

https://en.wikipedia.org/wiki/Secondary_air_injection

EDIT: OP drove an older truck. In earlier days, the extra air injection into the exhaust was to provide some air for the secondary exhaust gasses to fully burn. It had to be done early in the exhaust where the exhaust gases are hot.