How does the interest rate for the SCB Jumpstart actually work?

For the first $20k, you will earn an interest rate of 2% per annum. Hence,

$20,000 x 2% = $400

For balance above $20k, you will earn an interest rate of 0.1% per annum. Hence,

$1,000 x 0.1% = $1

In any case, take note that this is the prevailing rate and is subjected to change by the bank.

Here is everything about me and what I do best.

You will have 2% interest on your 20k, anything in excess will earn the prevailing interest rate. So like Hariz mentioned, I'd move my excess funds to another account.