The key is not letting your losses affect your bet amount. With the gain being only 80% instead of 100%, betting your bank means 1 win and 1 loss leaves you with less than you started. Making your bet amount fixed between flips means 1:1 will instead give you a net gain. The Kelly Criterion says there is an optimal proportion of bank you can bet that will maximize this gain over many flips
The key is not letting your losses affect your bet amount. With the gain being only 80% instead of 100%, betting your bank means 1 win and 1 loss leaves you with less than you started. Making your bet amount fixed between flips means 1:1 will instead give you a net gain. The Kelly Criterion says there is an optimal proportion of bank you can bet that will maximize this gain over many flips