STL309: Everyone needs a planer sled

Question 1:

From Jon:

Thanks for all the great content as always. I’m having some confusion about planers and planer sleds I am hoping you can clear up. I have a Dewalt DW735 planer and it works great. I do not however have a jointer so I have been spending an excessive amount of time flatting boards on planer sled that is basically just a piece of MDF. It works fine but it the set up time really adds up. I have kids and a job and very little time in my shop/shed. I was researching other planer sled designs and came across one that came from Woodsmith.  Fine Woodworking also has an article with a similar sled from Keith Rust in October 2005. This sled design uses adjustable cross beams, spaced out 4”-6” to support the board from beneath so the top of the board can be flattened.

My question is, why is the force of the rollers…