Write Spanish words with the correct accent mark (tilde). The drill is accent-strict: café is right and cafe is wrong, because in Spanish the accent changes how a word is said and sometimes what it means — tú and tu, sí and si.
Words you misspell come back sooner until the accent is second nature.
Every card has a temperature. Get it right and its chance of coming back halves; miss it and it doubles. K2 picks the next card at random, weighted by that chance, so a session keeps returning to whatever you are getting wrong and lets what you know fade into the background.
When the cards you are working on have cooled, a new batch joins them, so you always study at the edge of what you know. There are no review dates and no daily queue to fall behind on. Every drill is free and works without an account, and everyone gets 20 free AI uses a day; past them, signed-in AI practice runs on your Bike4Mind credits.