  We introduce a generalization of the concept of a chronological list of
forces, called a relaxed chronology. This concept is used to introduce a new
way of formulating the standard zero forcing process, which we refer to as
parallel increasing path covers, or PIPs. The combinatorial properties of PIPs
are utilized to identify bounds comparing standard zero forcing propagation
time to positive semidefinite propagation time. A collection of paths within a
set of PSD forcing trees, called a path bundle, is used to identify the PSD
forcing analog of the reversal of a standard zero forcing process, as well as
to draw a connection between PSD forcing and rigid-linkage forcing.
