I hope you enjoyed it and thank you for your feedback!
In the second level, where that headbump mechanic is introduced, the blue player is stuck in a very small space until the players both use that technique. I figured that, at the top of the level, the red player will venture out - but then realize the blue player is stuck behind the blue wall. The red player, free to pass in and out and not seeing any way to help on the outside, will go to the blue player, I figured. In the likely event neither player realizes the headbump mechanic initially, I figured they would inevitably jump on each other in their attempts to hop the blue wall, being that the space is so tiny. From there, the players will excitedly discover this new technique, which is needed (and reinforced) shortly after in that second vertical challenge in the same level.
The two people I had playtest the game said that the headbump mechanic (or any other parts of the game) did not need explained in an additional way. I do feel like that mechanic is more natural to discover with two players, so I'm curious if that would have been a lot easier if you played it with a buddy...
That's awesome you can round up a great number of playtesters! In future jams I'll try to have more - sharing it online is a good idea.