cindertransfer moves files directly between two browsers over WebRTC. Cinder's own role is small and temporary: it carries a few hundred bytes of connection handshake — twice, at most — and once that connects, the file itself moves peer-to-peer, encrypted end-to-end by WebRTC's own DTLS. It never touches cinder's server, never counts against a size cap, never sits in storage waiting to be picked up.
Both sides agree on a shared secret — typed by hand, or paired automatically: each side generates a one-time key and you both derive the same secret between you (ECDH), so nobody ever transmits it and it's never reused for another transfer. That secret decides who connects to whom; from there, it's a direct browser-to-browser channel until the transfer finishes.
Not a file host — nothing is ever stored anywhere by cindertransfer itself, on cinder's server or otherwise. If both people close the page mid-transfer, it's gone, the same way handing someone a USB drive in person would be if you both walked away.