I believe if fly or continuous sparging, you'd slowly drain as you slowly filled with 170F sparge water up top. Some flyers may add some hot water first to raise the mash temp like a "mash out," but some do not.
If batching, you'd drain the 154F as your first runnings, then add 170F, let sit awhile, then drain it as 2nd runnings. Again, some batchers add some hot water to raise the temp and/or to equalize the volume of the two runnings.