This will depend entirely on how you have Tomcat setup. If you have it setup to reload classes, then every 15 seconds or so it should reload changed classes, if you have reloading turned off (sometimes people prefer this for production machines) then it will never reload so hotsyncing won’t work.
Hotsyncing is a two step process, 1) MyEclipse gets the new file to your app server and 2) Your app server needs to d osomething with the new file.