Bridge Assurance is only used with RPVST+ and MST and only on point to point links. Bridge Assurance modifies rules for transmitting BPDUs. When it is activated on a port the port will always send BPDUs each and every Hello interval, regardless if it is a Root, Designated, Alternate, or Backup port role. BPDUs basically become Hello messages between pairs of connected switches.
Bridge Assurance ports are required to receive BPDUs, if none are received the port will be put into a BA-Inconsistent blocking state until the port begins receiving BPDUs again. Bridge Assurances helps to ensure a loop free topology against loops that are introduced by malfunctioning switches when they stop participating in RPVST or MST.
Bridge Assurance can also be activated globally and applies to all the relevant forwarding port roles, the neighboring device must also be configured for Bridge Assurance as well.