Some questions

[micmicmic]

I learnt a phrase recently, that is to lay it on the line. I understand that means to be honest, to tell the truth without euphemism

but what i really want to do is there any meaning for on the line ? or it's just practice or english usage.. as i am a non-native speaker so i don't really understand why these words make the sense of to tell the truth without euphemism


Also, i would like to know the meaning behind the proverb
tell me and i will forget
show me and i may remember
involve me and i will understand

thanks in advance!

[Tdol]

There's a debate here on the subject: Re: "lay it on the line"
There are a few theories.