These are odd creatures- they're not Xen entities, but are in fact made up of a trigger_push and a func_door, and are very simple to make. First of all, make the crater shaped bounce pad using the appropriate textures on a chopped cylinder or crater, or some other shape.

Create a trigger_push entity over the top of the pad (see picture on left), which pushes upwards at a fairly high speed (on the 'Bounce' DM map, this was set to around 3000) but leave the flags unchecked.

Now, underneath the bounce pad, make a cone shaped (pointing upwards) func_door. It should be made so that when the player stands on the pad, it looks like it is 'springing' up and forcing them into the air- basically, it's function is to push the player into the trigger_push, and to give the visual effect of the launch.

Make a trigger_multiple covering the bounce pad, targeting a multi_manager. This multi_manager should in turn trigger the func_door, as well as an ambient_generic that plays doors/aliendoor3.wav.

I have no idea way, but trigger_push and negative gravity won't make the player go up until they are given a tiny push, like the player jumping or a func_door or func_plat.

Tutorial updated. The key is really to have a trigger_push acting upwards, so it fires the player into the air when they jump into it or are forced into it by the func_door underneath.