Deepfly is a fly between an unfrozen tee and a deep frozen tee. When the unfrozen tee hammers the deeped tee, the deeped tee is granted a single frame of input, in which it can hammer back at the unfrozen tee. Although unstable, deepfly is functional and can have many uses.
sv_deepfly server setting, which means not all maps have deepfly enabled.
Performing a speedfly is possible with deepfly.