Yeah says the effect not the summon though. I would think that the person should be able to react to it being summoned before the effect happens. Can it prevent CV from canceling its summon as well?
The thing is, when a monster has a "when Summoned"-effect, that effect triggers immediately upon summoning before the opponent is allowed to trigger a trap such as Floodgate Trap Hole (Counter Traps such as Solemn Judgment are the exception). In other words: while it's true you react to the summon, you don't chain to the summon but instead to the effect.
There's a thing in Yu-Gi-Oh! called priority, the turn's player always has the priority to activate his/her effects before your opponent when both effects reach their requeriments at the same time.
Solemns Counter Traps and any other summon negation cards or effects are activated before the monster is properly summoned, they're totally different from Floodgate Trap Hole or any other cards or effects that respond a monster properly summoned.
Thanks for the info, how do people play this game with the actual cards. In game all the windows are done for you. Does the player have pause every time he does something until the other player is like "Go ahead"