They bump a joint into the old pipe, restrain that joint and bump a new joint until it seals, then remove the restraint and push the newest pipe into the old now encasement pipe moving the restraint to the last joint pushed, repeating the process until the pipe is down to the next line segment or manhole. essentially using the old pipe as a preexisting bore hole. Its less invasive pipe bursting, or bumping bore pipe without having to actually bore anything.

Thank you!