Simple as that, when you collect 50 rings in a stage with a signpost, it will spawn a large ring above the sign post and when touching it will take you to a special stage, however if you lose your rings the ring will disappear. Better store them with care! If the stage doesn't have a sign post, then no special stage 4 u.
This mini mod is still a tad buggy as multiple signposts will produce the same trigger sounds and can also allow you to access the special stages multiple times (like a token), it also acts buggy with tailsbot.
Credits to Lat' for allowing me to rip this out of SRB2DVD so that getting to the special stages are a little easier to obtain.