I agree that this is not the correct technique, but ...
1) My aspirator has a backflow valve, I can safely turn it off, the system will keep the vacuum, no water suck back.
2) With the pump, there will be suck back, but it takes sometime, usually I turn off the pump, then fast, disconnect the hose. Once I forgot, I got pump oil sucked over my ketone. Easy to separate, though.
Indeed, I agree with the comments, I am wrong, sorry.