In SWOLF and Swimming Efficiency I suggested a test set of 8×100, as fast as possible, holding a specific number of strokes per length (SPL), to hone in on your most efficient combination of stroke length and tempo.
I frequently do a modified version of this set as a quick tune-up before a competition or a challenging distance workout: 12×100 short-course, aiming for the following SPL on each rep: 15, 14, 13, 12, 11, 10, 10, 11, 12, 13, 14, 15. Moderate, controlled pace on all - no more than 75%.
Obviously, the specific SPL goals will differ for each individual. For me, 15 SPL is my 400m/500yd race pace. 14 SPL is my 1-2 mile race pace. 13 SPL is my marathon pace.
The reason I like this set as a warm-up / tune-up is that the act of “depriving myself” of one stroke-per-length on each of the first 6 reps really focuses my attention on efficiency - maximizing the amount of water I’m pulling, and minimizing drag. Then, adding one SPL on the way up (11, 12, 13, 14, 15) feels increasingly luxurious and powerful.
The over-arching goal: the 13, 14, and 15 SPL reps on the way up should feel better, faster, and more efficient than the 15, 14, and 13 SPL reps on the way down.
I took some GoPro video of myself doing this set, so you can see the subtle differences in my stroke from one rep to the next. The video shows a bottom-of-the-pool view of me descending from 15 to 10 SPL; then a side-underwater view of 14 SPL; then an above-water view of 14 SPL.