it is not a complicated zone. but it is tough to utilize effectively against quality 3pt shooting teams (esp from the corners). it requires great anticipation and rotations and *angles* on closeouts from the defenders. and those 3 things in the previous sentence definitely take a lot of time in practice for new guys. especially in the modern era of 3pt shooting, against weaker teams, when giving up too many 3s this zone (or any zone) can keep teams within striking distance. but this zone is not complicated. it is mostly a "23" zone, very similar to boeheims. but it is masked with a 131 setup. pastner calls it "20." they stack the guards, they elevate the wings. that is what makes it look like a "131" and different from a "23". but the responsibilities/rotations are very boeheim "23". on entry passes to the wing, the guard on the nail bumps the wing back (just like boeheims "23") while the guard at the point drops back to cover the nail. like boeheim, strong emphasis on preventing passes to the nail. on quick ball reversals that eliminate the wing bump on the weakside, the 5-man on the baseline has to rotate out to cover the corner 3 sometimes, and imo this is the toughest rotation in the design of pastners zone.
i hesitate to call boeheims zone a "23" because if you go based on the intial setup, boeheims is not a "23" - but because it goes back 30-40 years, everyone just goes with that terminology. boeheims "23" at the start of every possession is really a 4-1 or a 1/4 ct flat 2-2-1. pause the tv as the ball crosses hc in cuse games and take a look - 4 zone defenders on the arc, one very long and athletic big in the paint, daring teams to go to the baseline and short corner. so for both boeheim and pastner, it kinda begs the question: who cares what it is called or labeled? doesnt really matter except that the inner workings of the zones for most coaches align with certain historical responsibilities/rotations, which is why ours is mostly/primarily a "23". (some will say "no way that is a 23" but the rotations, bumps, etc are very very "23".)
pastners can be a little confusing on the surface because occasionally he goes traditional 131 with much different rules/rotations (traditional 131 zone that everyone learned in middle school). for GT, you know the difference before the ball crosses hc. instead of 2 guards stacked up top, and a 5-man on the baseline, as is done in pastners "20" defense, in his "traditional" 131 we put a forward at the point, the 5-man on the nail, and fill the other 3 spots accordingly. last year we very rarely used this. barely at all. most games we never used it. when we did, it frequently led to quick easy buckets and pastner switched out of it after just 1 or 2 possessions. so far this year we have mixed it in more often, but still nothing close to the hybrid "20".
side note: the "tell" for color analysts on TV is how they describe our zone. easy to see who has done their HW vs who is just making **** up. last year bilas was the best i heard at describing it accurately. he described it as a "hybrid" that starts as a 131 and then switches to a 23. also last year, one other guy (dont recall his name, only that he was a former cbb player and new to doing TV so they only gave him the streaming/online games) also nailed the description and details of it. i immediately wanted him to cover more of our games. separately, i really like babul on our games, he is far better than most of the national guys, and not just because he is a GT alum, he is just good at not talking too much like dan bonner et al... but i was disappointed last week when babul did not know how to describe or breakdown our zone.