Sans will follow her direction, jumping the three of them forward a hallway at a time, always moving them in the direction they are facing to avoid disorientation. It's basically like popping around on Google Street View - fast forwarding through the part where you have to walk through grimy water, and where Sans has to put any strain on his increasingly sore spine.

If Reira's directions hold true, soon enough they'll be back in the area Reira started, though Sans is feeling unnaturally worn out from the effort of teleporting.