A: We do a lot of X-Rep variations. We’ve even tried ones similar to that. For example, on leg curls we’ll do a set to failure, get help to the contracted position and pulse there; at X-Rep failure we lower to the midpoint and pulse there. We finish with X-Rep pulses at the max-force point, near the bottom of the stroke. The problem with that (and your version as well) is fatigue. By the time you reach the max-force point, the most effective spot for fiber activation, the muscle will be riddled with lactic acid and not be able to fire effectively. Our preference is to do the first phase of a drop set with X Reps at the max-force, to hit that key sweet spot with as much intensity as possible. That takes full advantage of the size principle of fiber recruitment as explained in The Ultimate Mass Workout. Then we’ll do the second phase of the drop with contracted-position X Reps at the end or an X Fade. If we do a second set, we may experiment with different variation.
So the answer to your question is, we’ve found it’s best to hit the max-force point when fatigue isn’t red-lining. It’s the most important point on the stroke, so give it priority most of the time.