Imagine that your brain is a company composed of several different departments, each with a different primary function. In any organization, there has to be some communication between departments to get things done. So, for example, the devops department has certain people in charge of working with billing to make sure they can afford and purchase new equipment, the billing department has people that communicate with the accounting department to make sure the equipment requests are in budget, and everyone has an HR contact to make sure they can hire who they need. In a well-functioning organization, these points of departmental contact are typically limited to a few people who are well versed in communicating between teams so there's not too much cross talk. But in some organizations, maybe they have a looser structure, and suddenly devops is getting mixed signals from billing because bobby went ahead with the approval while daren checked with accounting and the budget is out of whack and accounting's oversight procedures fail or don't exist because HR didn't give them the right head count because HR solely consists of brenda who is overworked and underpaid.
So with the brain. There is an optimal number of cross connections between systems to balance flexibility (how quickly devops can get equipment approval) with stability (budget oversight from accounting). Sometimes there are too many connections (bobby and daren), while other times there can be too few (brenda). In ADHD brains, the overall connectivity between systems is higher that this optimal point. This causes a breakdown in this balance toward hyperflexibility (usually - the nuances there are best for another conversation) because target systems are receiving mixed signals a la bobby and daren.