Portfolio loan payment

[Reggie Rearden]

I met with a local lender today regarding a multi family property. He advised me on the possibility of a portfolio loan. He will do 15 to 20 year amortization with a 5 year term. I'll quote what he stated. " The payment is calculated over 15 years. The payment would not jump around. It would be as if it were a line of credit. lines of credit are interest only and payments are based on the amount you draw." 

My confusion is if it an interest only payment, how does the principle get paid down? For example, if I have access to $200K, do I pay interest on that $200K even if I don't use it for 6 months? if I use the entire $200K, will my payments be that same as if I didn't access any of it? if I use $20k immediately and use $50k a year from now, will my payment adjust to reflect the new balance? For some reason im not grasping this portfolio loan re-payment concept.