You're on the right track with the timer interrupt assumption!

One of the key techniques involved the use of the Intel 8253/8254 Programmable Interval Timer (PIT), present in IBM PCs/compatibles at the time. DESQView would reprogram this timer to generate interrupts regularly, and more frequently, than its default setting used by the operating system for clock ticks.

When this timer interrupt fired, DESQView's interrupt handler would kick in, save the current state of the CPU (registers etc), and then switch the context to another process by loading its saved state. By juggling the CPU time between different programs this way, DESQView could give the illusion of running multiple programs simultaneously on hardware that was originally designed for a single task at a time.

DESQView used other tricks too, like monitoring keyboard and mouse interrupts, to make multitasking smoother. Pressing the Alt key by itself brought up the DESQview menu, and pressing Shift+Alt allowed you to record custom macros. A more interesting thing was how DESQView managed to intercept Ctrl+Alt+Del - pressing it would only terminate and close the window you're in, instead of rebooting the whole box, which, at the time felt like black magic because normally nothing intercepted Ctrl+Alt+Del - you expected it to just work and reboot the box without question.

cc: @raduzaharia@lemmy.sdf.org