I have to admit, the teaser have not been my favorite Pilates exercise. Ever.
I had, for the longest time difficulties to get and keep my legs up there. Please note I have very, very long legs ;-)
It's not an excuse, but I feel it has been my psychological block, together with a slight actual physical disadvantage.
Some years ago I used to have to modify with legs in a open bent "diamond" position, which still got my power house working and did not involve the psoas. I still use it if I am very tired and feel my legs falling down. I rather focus on my power house in that position, than strain my legs and use the wrong muscles to fight them up. Not constructive.
Today and some times earlier in the fall, my excellent Pilates instructor Adrianna, had me doing the backstroke into teaser. It's so much fun. The backstroke is then done further back on the box, almost as far back as for teaser. As one reaches out on a diagonal, after the reach up and circle around, one simply reaches up and out into a full teaser :-)
I adore it!
It also seems to completely eliminate that my legs work, and enables pure powerhouse engagement. You've got to love that :-)