Depending on your machine SP Game chip, there's a credit limit, and a hopper limit. I'm assuming you set the hopper limit to 400 since you say you can pass the 400 credit mark.
On some SP Game chips, the credit limit will equal the hopper limit, in this case any pay that will take the machine over the hopper/credit limit will be paid out by the hopper. For example. Say your hopper/credit limit is set to 400. You have 390 credits on your machine after the wager. You spin, and win 30 credits. All 30 credits will be paid out by the hopper. You spin, and win 10 credits, all 10 will be paid by hopper. You spin, and win 9 credits, they will be added to your credits. You spin and win the top award, you will go into hand-pay.
On the SP game chips that can have a different hopper limit and credit limit, things are slightly different. Say your credit limit is 1000, but your hopper limit is 400.
You have 390 credits after the wager. You spin and win 30 credits, they will be added to your credits. You hit cash out, your credits read 420, you will go to hand-pay. You spin and hit top award, you go to hand pay. You hit a win of 800 coins, you will go to handpay (since the 800 will take you over the credit limit AND the pay is higher than your hopper limit). You have 990 credits, you spin and hit 30 credits. The 30 credits will be paid out of hopper.
I hope that is clear enough to understand.