Your S+ input always starts out to be coins or cash.
If we assume a 3 coin machine you can put in 3 coins, each add a credit. Once 3 coins go in, the rest would fall to the coin tray.
If you have a bill validator - you put in a $20, this would usually put 80 credits on the machine (assuming its setup for a .25c denomination)
Depending on the configuration of the machine you might get 3 credits and the other 77 fall to the tray.
When you play (and win) the coins either get paid out or go to the credit meter. The Cash-Credit button generally determines this.
These behaviors can be changed.
Take a look at Ricks Faqs (bottom right of the home page) - there is a sequence to determine what Game Chip you have.
Different game chips have different options. Post what game chip you have and we can provide more specific instructions.
One game chip in particular supports a Montana Credits which allows you to stuff your machine with coins and have the credits rack up. This was not allowed in most gaming jurisdictions.
You can set the Cash/Credit that all pays go to credits, when you cash out you can set your hopper limit low so not too many coins come out before the machine goes to hand-pay.