could anyone please tell me how I can do the disappearing effect in Motion
I'd like to do, for example,
in the same scene, a man jumps up and then disappears
not using smoke mask or anything else
the man just disappears
Unless you're using green screen to composite the man over a background plate, the easiest thing to do would probably be to switch between one shot of the man jumping and another shot of the same location without the man in it at all. If the camera is set in the same spot for both shots and lighting, etc. is the same, it will look like he disappeared.
Or, if you didn't shoot a clean plate (meaning the identical scene without the man) you can usually find the same small area the man will disappear from before he goes there. Duplicate the video and crop to that area over where you want the man to disappear. This assumes that the video is not in motion and was shot with a locked-down camera. If your camera was in motion, its much tougher but still doable to isolate that area clean and keyframe it to track with the motion of the master shot. I'm guessing you're only talking less than 30 frames before the shot changes.