  A striking general bound on the energy gap in topological matter was recently
discovered in Ref. [Onishi and Fu, Phys. Rev. X {\bf 14}, 011052 (2024)]. A
non-trivial indirect derivation builds on the properties of optical
conductivity at an arbitrary frequency. We propose a simpler derivation,
allowing multiple generalizations, such as a universal bound on a gap in
anisotropic systems, systems with multiple charge carrier types, and
topological systems with zero Hall conductance. The derivation builds on the
observation that the bound equals $\hbar$ times the ratio of the force by the
external electric field on the charge carriers and their total kinematic
momentum in the direction perpendicular to the force.
