Over the decades, I cobbled together a repertoire of drills from various classes in karate, kickboxing, and a bit of boxing (very little). I cycle through them, but sometimes I come across something that seems to be sacrilege.
One such combo is to expand one's stance forward by stepping forward with the front leg and throwing a straight punch. I hesitate to call it a jab because the chin isn't shielded by the shoulder, and boxing drills never have you expand your stance. The follow-up is a reverse punch (again, hesitate to call it a cross), but notably, with the sacrilegious dragging forward of the back foot.
According to any art, that doesn't make sense because you're back foot isn't driving the cross. But sometimes, some moves that don't appear sensible at first do have a reason. For example, from a long-ish stance, you can step the back foot forward by half a step while jabbing, but it's more of a distraction so that you can drive forward a reverse punch from the back leg, which has been brought closer to the target.
Before I discard the dragging forward of the back leg during a cross, I was wondering if there is any situation in which this would be useful? Because of the dragging forward of the back leg, one is in the finishing position of a cross rather than the more extended position of a reverse punch. The follow-up to that is a simultaneous soto-uke (inward parry) and leading foot sweep (and there's more follow-up after that). Perhaps this is in response to an counter-attack. In the same way that one is loaded for a hook after a cross, one is loaded for the soto-uke and sweep after doing sacrilegious dragging forward of the back foot during a cross.
I only described the 1st two components of a longer combo. There's no realistic expectation that these moves will actually ever be executed in this exact sequence, but drilling in such combos does help with transitioning between moves. The leading straight punch and the following sacrilegious cross can be a mixture of high and low.
One use that I mulled over was that the sacrilegious cross baits the opponent's counter-attack and distracts in order to improve the success of the subsequent sweep. It could more of a distraction if a lands, e.g., fazing the opponent to improve prospects of the sweep. Since the back foot is coming forward, you have more range with the sacrilegious cross, which could provide a surprise factor. If you punch through with your body moving forward, it be quite a good fazing, especially with hip rotation.
Of course, one can rationalize weak moves, but it depends on the practicality. One example of a fake that I see as likely is in my third paragraph above (leading jab with a half step forward by the back leg, which better positions you to drive the following reverse punch forward).
Another possible saving grace is if one were to plant the back foot on the ground upon impact of the cross. This assumes that the cross travels forward synchronously with the dragging forward of the back foot. It would have to be quick slide rather than a slow drag.
Looks like it's not sacrilege in all circles: https://www.youtube.com/watch?v=jiVgEWAdIIA&t=215s