True dumping is the after sugar effect. It is very similar to what diabetics get.
Post-op bypass bodies process sugar rapidly and this can cause the dumping effect. Broadly speaking - heart racing/palpitations, nausea, clamminess, faintness and the urge to sleep it off.
I have dumped around 8 times in 2 years and boy, does it come on quickly and last hours!
Sugary stuff is best avoided if you find you are prone to dumping. Unfortunately, you won't know if you are prone until you dump!