Exactly right. There’s a small hole in the free end that creates enough restriction for the bladder to inflate but lets the water come out once it’s sealed in the pipe
I had a stubborn clog like this. Draino did nothing. Drain took an entire 25ft snake and found nothing. I went and bought a drain bladder. You hook it up to a water supply and it expands to seal itself in be pipe and then uses pressure to blast the clog out. I figured I had a 50/50 shot at either clearing the clog or exploding a pipe in my wall.
I have no idea where the clog was or where it ended up, but I haven’t had a problem in the 2 years since