When Don and his dad went to install that first piece of subfloor, they discovered that they needed to do a little bit of work first. See, when they sistered the new joists in, the old floor was sooooo un-level that sometimes the top edges of the new joists were above the old ones, and sometimes the old ones peeked over the edge of the new ones. So all of the places where the old joists were higher than the new joists had to be planed down. Rather than going and getting a power planer or anything sensible, Don decided to use the reciprocating saw to just slice off the bits that needed to go.
The process looked a lot like leveling out a cake for decorating. Just... noisier.