See this video for a clearer explaination and visual on how to recreate.

Basically if my head runs into a block I found that I fell slowly as if I was attatched to the block.  To recreate place a block above in front of you at head level and walk straight into it.  Make sure there is space below to fall.  I found I could recreate it more frequently in the video pushed up against a block before walking straight into another.