Once you unlock the B-Side by finding the Cassette Tape in the regular level you will open this level variant. The B-Sides are much harder versions of the levels and do not contain any collectibles. At the end of the level there is a yellow heart which you must dash through to complete the level. Use the collectible guide contained in this achievement solution to find the hidden cassette.
After dying 1,300 times to complete the main game I was ready to fast track this and get it done. Turning Assist Mode On allows you to activate unlimited dash and invincibility. It makes the game a bit boring as there is no challenge but it helps make for a quick way to collect missing collectibles and get through difficult sections. All achievements unlock normally with assist mode on so light it up and have fun spamming dash!