Push-pull welds are generally series welds which make two welds at a time. In some cases only one weld is required and there may be cosmetic reasons to keep marks off the part. In this case the second pair electrodes are flat and large. They become electrical contact pickup points. The same dual transformer arrangement is set up with the proper polarity but one set of electrodes are flat not contoured. This is an indirect push-pull weld.
Only one weld is made in an indirect push-pull weld. There will be very little marking on the part at the pickup points. You do have to have access to both sides of the part for this process.
Reference: RWMA: Resistance Welding Manual, Section 1, Chapter 2