Circular saws (also known as a skill saw) are not often used with wood floor installations unless you're a penny pincher or the wife doesn't allow other tools in the budget. It could be used for end cuts, but is very unstable and not recommended for safety reasons. Other times a circular saw may come into play would be during reconstruction or turning carpeted stairs into hardwood stairs. In this case it is used for removal of the substrate bull nosing.
Plywood On Concrete Sub Floors
Installations that require plywood on concrete find the use of a circular saw when cutting sheets to fit in areas. Additionally the saw is used for kerf cutting the backside of plywood if the subfloor is being glued to concrete. Often called relief cutting, this procedure allows plywood to settle into adhesive more effectively while bending somewhat to the slight contours of the subfloor.
Good For Board Replacements
Circular saws are most often used for board replacements when a damaged area needs to be repaired. The adjustable cutting depth allows for different thickness floor replacements. On a final note, the saw can also be used if you've found you've installed the floor too long. In other words, you have not allowed enough expansion area against a slider area for example, and the molding does not fit proper.
For specialty designs or patterns, the tool comes into use when installing borders around room perimeters. In many cases the floor is laid and installed over a preplanned reference point then trimmed squarely with the aided use of a guide or straight edge (below).
Below is an existing red oak floor that had a carpet inset removed. The job called for installing hardwood in the center on a diagonal with a nine inch border design.
Circular saw with straight edge
Saw blade types used best with this tool are 24 inch carbide tipped. It should be noted the circular saw should only be used for purposes it was designed for. I recall asking a guy years ago why he had this massive scar on his arm. He had tried to use the saw to undercut baseboard and it got away from him.
Circular saws can go through a lot of abuse. During periods when it is used for cutting borders, play close attention to the bottom plate or the area the saw glides along the floor with. Metal burrs can rear their ugly head over time, causing scratching on a new prefinished floor. Sanding off the burrs will be effective but most professionals will also protect the plate with blue painters tape, preventing such costly blunders.
Extension cord obstacles can cause serious job related injuries. When using this tool with longer cords make sure the area is unobstructed. Keep cords from untangled messes.