We have an old toilet cistern in one of our bathrooms that the wind howls through during the winter, chilling the room significantly and also presents a risk to the cistern freezing up. This dip end fits to the top of the pipe (For bottom entry overflows) and dips into the water which prevents the draught through the pipe. The pipe will still allow water to overflow through it should the ballcock begin to fail.
Most modern cisterns overflow into the toilet pan but if you have an older design with a bottom entry, this may be useful to you. It's designed as a loose push fit onto the top of the 21mm overflow pipe. It may be secured with a small application of silicone in the pipe socket but don't overdo it or you risk blocking the holes where the water exits.
Recommend printing in PETG or ABS due to the biodegrading of PLA in immersion of water.