The STP PortFast BPDU guard enhancement allows network designers to enforce the STP domain borders and keep the active topology predictable. The devices behind the ports that have STP PortFast enabled are not able to influence the STP topology. At the reception of BPDUs, the BPDU guard operation disables the port that has PortFast configured.
Yes, as a side effect of shutting down the port as soon it receives a bpdu;-)
There is a related feature called "root guard", that will block a port if it receives better information (spanning-tree guard root). Configure it on ports you don't ever want to become root port. Be careful that it's rather a security policy enforcement to be configured on port for which it would not be appropriate to become root port (like a port connecting to an end user). Don't use this kind of feature to select your root port among a group of candidate.