Originally Posted by Imagine That
My friend used to say if they repeated naughty behaviour when you are riding them, you should keep their tack on (Bridle,saddle,e.c.t) and tie them to a "Thinking Post" for half an hour or so, and that would be like a "Time Out".
I'm not really sure about this, what do you guys think. Will a horse understand why they are there?
Your friend is wrong. Horse will NOT understand the punishment like that, and the end result it may associate you with something bad coming.
As several people already mentioned: something is wrong - correct it right away. Of course given it's not a physical or health reason for the horse to behave off (like sore back, bad tack, teeth issues, etc.). If you don't know how to correct or doesn't feel confident, use a trainer to help you with such corrections.