An account is created for an e-mail address — the same address will receive the sign-of-life questions.
LegacyPing is a dead man's switch. It makes sure the people you chose receive material you prepared in advance, if you stop giving a sign of life.
You encrypt attachments before uploading and the server never learns the password. The recipient gets that password outside this system — in a sealed envelope. Without it the parcel is useless noise, and that is the most likely way the whole thing fails.