Deadline for Elective Deferral Contributions

Hello, I am self-employed with a solo 401k.  I am confused about the deadline for making elective deferral contributions.  I understand that I have until the tax-filing deadline to make the elective deferral contributions, but what does it mean that the contributions "must be elected by the end of the tax year"?  And how would I prove this?

 

Thank you,

Ryan