Abstract Side bileaders are commonly induced near a progressing positive leader, yet their effects on the parent positive leader remain poorly understood. Here we investigate their effects by combining high‐speed video with LF/VLF lightning channel mapping. Our results reveal that the approach of a bileader promotes needle‐like discharges from the lateral of parent channel, which act as connecting leaders. In eight out of nine side bileaders observed in two flashes, these connections significantly enhanced the luminosity of the positive leader channel on the trailing end from the connection point and formed a new positive branch. In one special case, the negative end of the side bileader propagated into high altitude (∼8 km) instead of connecting directly to the parent channel; its eventual connection increased the luminosity on the leading end of the parent channel and produced both new negative and positive branches. We propose a potential‐driven mechanism to explain these observations.