That works for many kinds of tasks, but not those that involve other people. For example, if you have to call someone it's not compelling to say "if I don't like talking to this person I'll just hang up after 5 minutes".
You could conceivably still make yourself an out for that. You could have an excuse for hanging up ready to go. Or, if you're talking in person, there are probably several apps that allow you to receive a fake phone call, and step away.