Conditional moving of card between lists

[James Quek]

I would like to prevent a user from moving a card to the next list unless a condition is fulfilled

e.g  all custom fields are NOT empty
e.g one custom field is set to "Yes" in YES/NO 

[Brittany Joiner]

I don't entirely know a way of blocking someone from moving a card, but you could use Butler that moves a card back if those criteria aren't met. 

Ie... make a "when card is in this list" and add filter for "custom fields blank", and an action of "move to x list".

EDIT: I just found this power-up (Workflows for Trello), I haven't used it so I can't really speak to it but it seems like it lets you set constraints for moving a card to a new list.