In this section, Horace and Jasper notice that the puppies have fled the room and start to track them down within the house. Pongo and Perdita meet the Colonel and head towards Hell Hall.
Shots 4 and 7 are fairly simple from a narrative standpoint, but difficult for animators. In each shot, Horace and Jasper move in depth, changing size as they move. The benefit of shots like these is to provide a sense of the characters existing in a real space that they have to move through. The challenge for the animators (Bill Keil, Amby Paliwoda and Ted Berman) and their assistants (unknown, unfortunately) is to maintain the proportions of the characters as they shrink or grow.
Even at the time, there were tools available to help the artists. The camera lucida (often referred to as the "lazy lucy"), an opaque projector with the ability to enlarge or reduce drawings, was often used. The artist would be forced to trace off the projection. As this film introduced the use of Xerox on a large scale, it's possible that the drawings were photocopied up or down. Most likely, several drawings were resized for reference and the artists brute-forced the animation to work.
This is the kind of thing totally taken for granted in the age of computer software. 3D packages provide accurate perspective by default and even 2D packages allow for resizing without requiring redrawing.
Shot 7 also includes some nice effects animation on the flashlights. They're double exposed for all the scenes they are in, but in shot 7 they occasionally are aimed directly at the camera and the double exposure is replaced by an airbrush glow.
I'm guessing that Hal King is responsible for Tibbs' take in shot 20. It's beautifully done. There's great shape changing on the fur and the arms and legs are pumping away on ones, leading to a fast vibration. Tibbs' landing on Jasper's face contains some great drawings by King and either Sibley or Nordberg handling Jasper.
There's a lot of puppy re-use within the crowd shots of puppies running. Shot 23, a low angle shot with the pups coming over the horizon to run down the stairs (probably animated by Berman) is a flagrant example. A limited number of puppies have been animated and then photocopied and repositioned in order to make up the crowd. If you look at the mass of pups in many of these shots, you can easily see the cheats.
In shot 30, (example below) take a look at how rough they left the drawings for the puppies. As they were all moving non-stop, somebody made the decision to just paint the roughs and hoped that the audience wouldn't notice. The puppy animation is probably by Ted Berman and in this case, we're looking at the animator's original drawings, not at the work of an assistant.