I've used several different sizes of the bladders in the past. They have a small opening on the end that also has a vibrating device in it. Any decent amount of hose pressure should expand the bladder to keep it in the pipe. If the plug is solid with barely any movement, the vibration will help break the clog. (If it's eight feet of solid grease, forget it!)
I have had drains that were impossible to get a cable through, and the bladder saved the day. OTOH, I recently helped with a job from a drain cleaning company where the bladder pumped water through a leaking vent and destroyed property beneath a leaking ceiling, so you have to use them with caution.
One problem with this type of pressure device is that you can put in a drain and the vent can fill with water, then when you shut off the hose the whole mess comes out of the wall. Also, a fixture below it can overflow flooding a basement or lower floor. You have to take great care with something like this.
