Momentary switches typically remain closed as long as the user pushes them. This is distinguished from latching switches that stay in place, such as push-on/push-off or lever switches.
What effect a momentary switch has when held closed depends on the way it is used - if it directly controls current to a motor or light the motor or light will continue to operate. If it is something like on a keyboard or mouse, then it is up to software to determine what is going to happen. At the least the controller that attached to the switch will typically make repeated readings to see if the switch is still closed and the software running on the controller can tell by timers how long it has been closed to decide if it should auto-repeat or just register as a single push and wait. This reading rate can be a hundred thousand times a second, which would make typing a real challenge if there was no built-in delay.
A similar situation is seen in cruise/speed controls on cars. One short push (and getting read a few thousand times) typically gives a + or - of 1 mph. Hold it for more than that and the controller starts adjusting the speed until the user releases the button. I would expect the trim to be operated in this mode.