The post, as originally designed by Tom Burczynski in the 70's, was in a soft all-lead bullet, and the post had a noticeable angle to the sides. I used to have the all-lead Hydra-Shok loads in both .38 Spl and .45 ACP. If I recall correctly, the angle of the sides of the posts were different according to caliber, to optimize fluidic forces acting on the posts, directed outward against the inside of the nose cavities, to maximize expansion.
Federal/ATK later licensed/adopted the Hydra-Shok design in the late 80's, using a jacketed bullet design instead of the previous all-lead bullet design. I remember looking inside the nose cavities of the new Federal Hydra-Shoks and noticing how different the posts were compared to my original Hydra-Shok Corporation Scorpian loads. They were much thinner and also straighter, to the naked eye.
It was later observed that the jacketed Hydra-shok JHP's could have their nose cavities plugged by materials (like enough dense cloth), which basically neutralized the ability of the posts to direct fluids outward against the cavity walls and which could prevent desired expansion. Since the bullets were no longer soft all-lead, the nose cavities could better resist deformation when encountering feed ramps (although I remember reading about at least one revision of the Federal .45 Hydra-Shok, supposedly done to allow better feeding in older .45's). I lost track of the revisions and changes of the Federal Hydra-Shok design sometimes discussed, but they eventually decided to keep the line in the mid-2000's (according to articles), as well as their HST line (which became the low cost LE ammunition line).
Basically, when the Hydra-Shok nose cavities didn't get plugged, the design worked to aid expansion, but the post of the jacketed bullets aren't quite the same as the posts used in the original all-lead bullets. The soft all-lead .45 Hydra-Shoks could experience deformed nose cavities during feeding, but that same softness, huge post and ease of being deformed earned them a reputation for huge expansion back in the day (if the gun would feed them).