The real Meat Boy, from your copy. Each time you press Play, his animations (idle, run, sprint, jump, fall, wall-slide, landing), his sound effects and Super Meat Boy's soundtrack are read from your own Super Meat Boy install. Nothing from either game ships with the mashup.
He moves like Meat Boy. Faster running and sprinting, a higher jump, wall-slides that leave a blood trail, and wall-jumps. No ledge grabbing and no fall damage.
Every heart is a life. Any hit splatters him with his Super Meat Boy death sound and costs one heart, and he's straight back at the level's start. On his last heart he dies for real and Spelunky's death screen lets you restart. Pets you save give extra lives.
Only SMB music: Spelunky's own music is silenced. The Forest plays in the Dwelling, the Hospital in the Jungle, Hell in Volcana, the Salt Factory in the Temple, and more.
Single player. Needs Spelunky 2 and Super Meat Boy (Steam). Melty installs it and starts it in one click.
Credits: Super Meat Boy by Team Meat. Spelunky 2 by Mossmouth and BlitWorks. Playlunky and the Overlunky script API by the spelunky.fyi team (MIT), bundled unchanged.