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. 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.
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.