I'd did the HM on a guy choking at a Backyard Burgers. The weird thing he was making these seal-like barks, which wouldn't seem possible with a blocked off airway. Anyway, it worked and it dislodged the hamburger.
I have heard of people's ribs being broken, and wouldn't be surprised if someone was sued after a HM. Fortunately I didn't hurt the guy, despite me being 6'5" and probably lifting the guy off the ground.
Most states have good samaritan laws that prevent such lawsuits from happening. The good samaritan laws do not cover professional EMS personnel however. As a professional, you can be sued for negligence if you render aid and you're found negligent.
Just as FYI / technical distinction, you can be sued for rendering first aid, the likelihood of that suit being successful is unlikely due to the protections under the good Samaritan laws. Lawyers can still be expensive. As a professional you should always have professional indemnity insurance on top of whatever the employer provides.
You can have an object lodged in your throat in such a way that it allows you to exhale but not inhale.
As a child, I remember choking on a bit of bread or pastry or something. I couldn’t breathe in, but I could wheeze and make noise. I was able to dislodge it by pointing my head downward and exhaling forcefully with my remaining air, effectively an internal version of the Heimlich.
I have heard of people's ribs being broken, and wouldn't be surprised if someone was sued after a HM. Fortunately I didn't hurt the guy, despite me being 6'5" and probably lifting the guy off the ground.