For every campaign you start you need credits. The cost of 1 view depends on your subscription plan, variable amount on the Free plan, 1.6 credits on the Starter plan and 1.2 credits on the Pro plan. Let's say you are on the Free plan and want 10 views, this will cost you 20 credits, 10 credits will be cost and the other credits are valid view credits.
When a user starts a player and your video will be opened, 1 of these 10 credits will be locked to this user + IP. The algorithm sets how long a view must be to be valid, this is a minimum and a maximum length, these are both random based on the time you have set. If the user keeps the player running between these to values, it's a valid views and your credit will be marked done and the viewer receives a credit for the view. If the user doesn't watch with the criteria, the credit will be unlocked so another user can view your video. So no lost credits, we validated every view.