2nd day of work and 2nd level new movement

Yesterday I got to ride Pablo, Falada, and Prince. Two Grand Prix horses and a 5 year old who is a sweetheart. Mike gave me some suggestions while I was riding Pablo, but then he got busy with Teenie, and then we went on a trail ride with George and Falada.
During my lesson, by which point my abs were getting tired (yay in long run!), we worked on:
- Foot position. Yes, toes straight ahead (as if standing) is preferred to 45 degree angle - for dressage, because you need so much fine tuned leg movement.
- We did a 3 loop serpentine for the first time. Very cool, but I need to think about it - I have to lift the inside seat bone - unless it's the outside - why I need to think about it. And the geometry, which looks simple on paper, is not as intuitive to ride.
- Collection and being on the bit. Willig was having a day where he really didn't want to be on the bit, so there was a lot of heavy handed riding by me - it makes my arms tired.
- But then we figured out, after a canter lengthening where I basically threw the reins down, that maybe what I'm missing working on the "poof" by myself is that I throw away the connection. When I keep the "heavy" contact, Willig LIFTS, which is weird and also counterintuitive because the poof is so light and easy to ride.
- Don't put up with the laziness - if I want him to respond immediately to my aids, them make him respond every single transition every single ride.
I was kind of pooped by the end of the day, although it was pretty much what I'd do every day if I was a millionaire and didn't have to work, so it was a very happy, good exhaustion. It's also soooo cool to get to ride all those horses, and then about a million times cooler to have Mike there to give me little pointers. This is far and away the best use of my time I've made out of the last - 10 years? My life?
However, Willig is the LEAST fun to ride of that bunch. They're all different, they all look at the scary end to varying degrees, but I can't quite put my finger on what makes him more difficult. He's more ... wiggly.