  We resolve a significant controversy about how to understand and engineer
single-shot all-optical switching of magnetization in ferrimagnets using femto-
or picosecond-long heat pulses. By realistically modelling a generic
ferrimagnet as two coupled macrospins, we comprehensively show that the net
magnetization can be reversed via different pathways, using a heat pulse with
duration spanning all relevant timescales within the non-adiabatic limit. This
conceptual understanding is fully validated by experiments studying the
material and optical limits at which the switching process in GdFeCo alloys
loses its reliability. Our interpretation and results constitute a blueprint
for understanding how deterministic all-optical switching can be achieved in
alternative ferrimagnets using short thermal pulses.
