For those not familiar with such things, HCSA is an account into which you pledge a certain amount of money annually. That money comes pre-tax out of your paycheck and goes into the special account. The good thing is that you get to draw on the entire amount on January 1, but you just pay a certain amount each paycheck. (In my case, 1/50 of the pledge amount each week -- I get slightly larger paychecks the last two or three weeks of each year.) It's all pre-tax money, so you're effectively getting a 20% or so discount on everything. You can spend it not only on co-payments and deductibles, but also on certain non-prescription items that ordinary insurance doesn't cover. But you have to spend it all in the calendar year to which it is pledged. If you don't spend it, you forfeit the remainder.
The HCSA is a good thing, but the use-it-or-lose-it nature of the program makes one leery of putting in too much money, lest you be obliged to go on a spending spree in December to soak up reamining funds in the account before they vanish. So far I've never got that far and have ended up having to pay out of pocket during the last quarter or so.