curious about this interaction and hoping someone could help me out.

lets say a creature has hex(concentration spell) on it and they have a wis disadvantage. If i cast hold person which is also concentration does the hex effect drop before the hold person happens? or do they still have disadvantage until hold person connects? basically i am asking at what specific point do you lose concentration on one spell before it kicks in on the new concentration spell?

thanks

Technically you lose concentration when you start to cast the next concentration spell. And it doesn’t matter for saving throws. Hex only affects ability checks, not saving throws. 

You would drop the concentration to cast the second one. So your hex ability would dissipate, you cast hold person, hold person takes effect.

Quote from Tim >>

Technically you lose concentration when you start to cast the next concentration spell. And it doesn’t matter for saving throws. Hex only affects ability checks, not saving throws. 

 True. You need to use bestow curse (available to warlocks via invocations) to impose save disadvantage. Note that curse, unlike hex, loses the concentration requirement when cast with a level 5 slot.