When can you end concentrwtion?

[Mellack]

This came out of a different thread talking about being grappled, but the game has many ways of forced movement. What if you are about to be forced into your own AOE? You don't want to face your own wall of fire/cloudkill/spike growth/etc. So when do you let a player end concentration? Is it instantanous, or only on their turn? Can they choose to fail a concentration check?
Situation one: wizard has a wall of fire up and gets hit by a thorn whip. Can he drop concentration before being pulled through?
Situation two: wizard casts web. Bad guy gets out and moves behind web. Can the wizard drop the spell so rest of the party have a clear path or does he have to wait for his turn to come back around?
I am interested in how other groups rule as I don't believe there is a RAW answer.

[pwykersotz]

PHB 203: You can end concentration at any time, no action required.

[bid]

Dropping your concentration should require using your reaction. RAW, there are no other way than the ready action to do such a thing. Your "fail a concentration check" idea implies you are taking damage already and might be too late for your purpose.

I think you are right: this has to be ruled by the DM/players.

I would allow dropping concentration or anything you hold as a reaction, but your party might think differently if it lets the bad guy to drop the princess down the ravine. There should be some minimal condition before you can take that free reaction.

[JoeJ]

I think RAW, as already referenced by pwykersotz, makes the most sense. Ending concentration doesn't require you to actually do anything, only to stop doing something. So no kind of action is necessary.