The attacker sends a stream of frames with an empty payload and without the end-of-stream flag. These frames can be DATA, HEADERS, CONTINUATION and/or PUSH_PROMISE. The peer spends time processing each frame disproportionate to attack bandwidth. This can consume excess CPU, potentially leading to a denial of service.

See https://groups.google.com/g/envoy-announce/c/ZLchtraPYVk, https://www.envoyproxy.io/docs/envoy/v1.11.1/intro/version_history and https://github.com/Netflix/security-bulletins/blob/master/advisories/third-party/2019-002.md.