(adsbygoogle = window.adsbygoogle || []).push({}); is it true it acivate a small bomb around u when u go down?

No.
But if you dolphin dive from any height, you will set off an explosion killing any nearby zombies.
Also you take no self-inflicted damage... no fall damage, no grenade damage, no ray gun damage.

when u dolphin dive from a decent height it does

when u dolphin dive u set an explosion around you and u take no fall damage

PhD Flopper is basically an explosion that you emit whenever you do a dolphin dive from an elevated place down to a ground-level area. The explosion kills the zombies within its radius. In my thread "Ascension Strategy" you can see what it does exactly.