It can do bursts of computation, memory access, or anything else that varies the amount you wiggle voltages or currents on wires in a way that emits radio waves. You can do it without even trying (which is one way some smartcards exposed private keys ...).
In the days of CRTs that applied especially well: Graphics output could modulate the beam and generate a LOT of radio. (Doing gray scales by making shifting fine patters would be an especially "in your face but you can't see it" approach.) A fast photocell could read it from the light, as well.
Preventing / shielding against things like this is what "Tempest" is about.
I recall, back in the late '60s / early '70s, when I was doing software on a machine at a classified site. It had a music program that worked by wiggling the lines on three console display lamps that were also connected, by three resistors (forming a cheap D2A converter) to a volume control T-pad and a loudspeaker. Turns out it also modulated the memory access and/or other signals - a lot. I had left it playing "moon river" overnight, drove up to the building, and heard it on my A.M. radio.
I realized it would have been trivial to exfiltrate a small amount of data, even on my starving student budget, by emulating an FSK modem and hooking a transistor radio to a battery-powered tape recorder (about the size of a briefcase in those days) left in the trunk of my car. (Not that I'd have needed to, since I could carry mag tapes in and out, but as a "white hat", how could it be done, exercise.)
The security guys figured that out, too. A bit later I got a ping from management: Some guys from Washington had also driven up, noticed the arcade-quality "music", and given them grief about it.