any PWM fan that receives no PWM signal will default to max speed. It's not due to the Splitty, that's just how all PWM fans work.
If you turn off the channel in the aquaero, there's no PWM control signal going to the fans, and they will go to max speed instantly.
The Aquaero gives them no power because it's an active splitter. it only takes PWM signal from the aquaero, and sends it back the speed signal taken from the fan plugged on the middle connector that's tagged as "rpm". You need one fan connected to that socket or the control won't work btw.
https://aquacomputer.de/handbuecher.html…ive_english.pdf