Elastic interest rate means that interest payed by borrowers on Kashi don't increase instantly when utilization goes up.
By using elastic interest rate we try to optimize utilization, the target is between 70% and 80%. When above 80% the interest rate will rise up to 1000% APR, bellow 70% it will start to drop with a minimum of 0.25% APR.
The APR evolution is determined by utilization and time, closer it is to 70%-80% and less it will change and the further is is higher is the evolution up to halves every 8 hours if 0% utilization and double every 8 hours if 100%.
Here is a chart explaining the interest change :