In some welding operations it is beneficial to apply a period of low heat to preheat the part prior to application of full heat. One option is to use upslope described in a previous article. Preheating is also used frequently. In preheating current is applied at some constant level lower than the final welding current.
The goal of this current might be to burn off contaminants, anneal and help seat the part or just bring the part up to temperature more gradually.
Reference: RWMA - Resistance Welding Manula 4th Edition