Not bad at all. Don't do it to a snake that is over 18". Then it can become bad. Just become educated, before you do it. It takes some pressure, but not a lot. Most people that do it wrong, aren't doing it back, far enough from the vent and rolling forward. You have to be a good 1/2 inch, from the vent and roll your thumb toward it.
I am adding to Cat Eyed Lady's post, just in case you don't have a weight.
Wayne