Don't ask me how because i'm still learning but... DRBD has 2 way to replicate:
Synchronous and Asynchronous
Synchronous mirroring (called protocol C in DRBD speak) is the right choice for HA clusters where you dare not lose a single transaction in case of the complete crash of the active (primary in DRBD speak) node.
Asynchronous mirroring is necessary to build mirrors over long distances, i.e., the interconnecting network's round trip time is higher than the write latency you can tolerate for your application. (Note: The amount of data the peer node may fall behind is limited by bandwidth-delay product and the TCP send buffer.)