That is quite hard as clear info on mechanics is rarely shared by developers in an easy to find way, but there have been some dev posts on the subject, which I mostly remember quite well. Searching for them is rather difficult, but I can tell you what I know.
When talking about the 'turn rate nerf' we mean the lowered speed of horizontal turning during an overhead or stab attack. Originally in Native this turn rate had a value of 14, which often meant being able to do like 120-160 degree turns during the animation, depending on the weapon speed. The first turn rate nerf halved this to 7, which meant you could turn more like 60 to 80 degrees. With a turn rate of 14 you were able to chamber lances and hit the horse in the ass if it sped by, you were able to hit any stab (1h, 2h and pole) at close reaches by starting it next to your target and dragging it into him, you were able to switch targets to someone nearly behind you. Turn rate nerf only affects horizontal turn rate and not vertical, hence why people have been able to still stab at close reaches by looking above their target and dragging it down.
The overall reduced turn rate to 7 affected fast and short weapons much more, because they got less degrees of turning due to a quick animation and because 80 degrees of turning at pike range covers much more distance than at steel pick range. This, combined with the fact that the nerf was mostly to restrict ridiculous roflcopter-stabs with enormous weapons, made a tweak necessary.
Much later this discrepancy was finally adjusted by making the severity of the restrictions based on length and weight, with a minimum turn rate of 5. The formula was posted once and I ran a few weapons through it that are important to me and I still remember the outcomes, to give you a vague idea of how it works. Great Maul, Long Maul, Pike and Longspear all went down to 5, those are the extremes on either side of the spectrum. Awlpike went up to 8. ±120 length greatswords went up to 8.5-9. Most 1h fell within a 10/12 turn rate region, which is fairly close to Native values. This made the turn rate nerf a lot more reasonable but there were still a bunch of weapons that got undeservedly fucked by it, so the devs were planning to assign turn rate values per each weapon. As far as I know this has never happened as it required going for full WSE 2, which apparently the community voted down. So this formula is the current status as far as I know, however various tweaks to stab hitboxes and animations have changed the effectiveness of stabs respective to eachother since then.
Edit:
Did a quick search and found the post by Paul with the formula, couldn't quote because the thread is locked, my memory seems to have served me about right.
http://forum.melee.org/general-discussion/this-double-hit-business/225/Currently the turnrate is 7 during an attack. Native is 14.
With the change it will be:
1h_turnrate =clamp(14-0.15*weight-0.06*(length-75), 7, 14)
2h_turnrate =clamp(14-0.14*weight-0.08*(length-90), 5, 11)
pole_turnrate =clamp(13-0.18*weight-0.04*(length-140), 5, 11)
weight in 0.1kg
Example: short sword [weight=10; length=81; 1h]
1h_turnrate(short sword)=clamp(14-0.15*10-0.06*(81-75), 7, 14)=12.14
Example: great maul [weight=80; length=68; 2h]
1h_turnrate(great maul)=clamp(14-0.14*80-0.08*(68-90), 5, 11)=clamp(4.56, 5, 11)=5
Later on there is a post from Pentecost with the turn rate values of a whole bunch of weapons
http://forum.melee.org/general-discussion/this-double-hit-business/255/Also found a similar poll like this one about 5 months after the original turn rate nerf
http://forum.melee.org/general-discussion/turn-rate-nerf-good-or-bad-for-the-game-in-general/msg673919/#msg673919