The forkball is a variation of the changeup. Forkballs very much mimic the splitter and are very rarely used by professional pitchers today. The movement is suited for advanced pitchers with a professional pedigree and it’s hard to explain the actions on how to properly throw it for that reason.
The tuck action almost mimics the path of the circle change, but normally has a greater drop off of the table. The forkball can lead to mechanical problems because pitchers tend to snap it instead of simply letting it ride out without downward spin. It sounds counter intuitive, but the stale release is the action that causes it to drop off the table. Since the circle change is an easily repetitive pitch with a great decrease in arm related stress, the forkball has become outdated.