This works well if the structural resonances are poorly damped. If they are well damped, you may even be hard pressed to get a responce with a bump test and will have to use some type of shaker. This technique also works very well on piping resonaces. Sometimes displaying in displacement units works to your advantage on this test or on bump tests. Rick Dear Rick, This is the same phenomena as I was discussing in my bumples bumptest notes.
Undamped resonances are easier to see.
The bumpless bumptest enhancement is just fallout from a process that was made to deal with rolling element bearing test problems. As I said in my original note, it's a semi-old trick. Just for fun, put your magnet based accel on the top of a vertical pump, in the horizontal direction and turn up the sensitivity on the analyzer. You will almost always see a peak bobbling up and down. Once you have the gain set so that the peak is easily discernable, bump the motor with the heal of your hand and see if that peak doesn't show a momentary jump.