Brute physical force = firmness IMO.
I don't sweat the small stuff, if stuff doesn't matter then they don't have to do it. But when I say 'You need to sit in the trolley while I do the shopping' I mean it and he will sit in the trolley. If he screams and yells and twists and tries to get out, tough shit.
Do I feel bad for other people who have to hear this? Yep. But I need to get the shopping done, I have the baby with me. That is life, I hear other people's kids, I hear adults making noise, welcome to a community.
DS1 was an easy going passive little dude, only had a couple of times with him yelling, DS2 can be a screaming demon when things are not going his way. Tough shit.
I agree with a previous poster that it is about confidence in yourself. I am absolutely sure that my way is the right way, there is no highway option so I make them do it.
I too feel for the PP who physically can't restrain her 3 year old, what a PITA that must be! I suspect DS1 now at 7 would take some restraining but it hasn't come up for years because we were able to early on.