on the old site there was the + - buttons on the side with the tags when looking at posts, which you could use to quick add or remove tags to your search. I used this pretty frequently, please add it to the new site.

I really hope these are added back. Seems absurd they weren't added already before launch, to be honest. Such a basic but useful feature.

These buttons are going to be added back to the site. Please be patient.

https://e621.net/forum_topics/25748?page=5#forum_post_286015