Once we are open to all our feelings they have a chance to be felt fully. Up to that point it can be hard to let them through, so now that they are allowed through, they are greeted with acceptance. This can go for positive or negative feelings.
The goal of acceptance is to not judge the feeling or try and get rid of it as quickly as we can. If we accept the feelings and allow them to be here they can pass through in their own time, rather than being stirred up by judgments or strategies about them. They are seen as not solid or real, but passing mental events.