$ for i in $(seq $(getconf _NPROCESSORS_ONLN)); do yes > /dev/null & done

Kill that via:

$ sudo killall yes

Park cores via:

$ for x in /sys/devices/system/cpu/cpu{1..11}/online; do echo 0 >"$x"; done

Unpark cores via:

$ for x in /sys/devices/system/cpu/cpu{1..11}/online; do echo 1 >"$x"; done

You’ll note that Core0 is not included, Linux cannot park Core0 unless you’re using a full no-tick kernel (which most Linux flavors don’t have yet). The best you can do is to use the low-latency kernel and the kernel flags in my comment here:

https://askubuntu.com/questions/185826/does-ubuntu-support-core-parking

To implement something as close to a full no-tick kernel (as Windows and MacOS use) as is possible under Linux.

On my AMD Ryzen CPU, with all but Core0 parked and with Core0 loaded to 100% (with Turbo Mode turned off), TDP is only ~3.8 W. For normal, everyday usage, I don’t notice all that much more lag than when all cores are running. That’s fantastic compared to the old CPUs that used to have TDPs in the hundreds of watts.

a=1
while true
do
let a=$a+1
done

Result: 100% cpu usage (single core).