It's not a command, it's a default preset configuration which looks like this:
High:
r_fullScreen 1
r_subdivisions 4
r_vertexlight 0
r_lodbias 0
r_colorbits 32
r_depthbits 24
r_picmip 0
r_mode 4
r_texturebits 32
r_fastSky 0
r_inGameVideo 1
cg_shadows 1
cg_brassTime 2500
cg_bounceParticles 1
r_texturemode "GL_LINEAR_MIPMAP_LINEAR"
Normal:
r_fullScreen 1
r_subdivisions 12
r_vertexlight 0
r_lodbias 0
r_colorbits 0
r_depthbits 24
r_picmip 1
r_mode 3
r_texturebits 0
r_fastSky 0
r_inGameVideo 1
cg_brassTime 2500
r_texturemode "GL_LINEAR_MIPMAP_LINEAR"
cg_shadows 0
cg_bounceParticles 0
Fast:
r_fullScreen 1
r_subdivisions 8
r_vertexlight 0
r_lodbias 1
r_colorbits 0
r_depthbits 0
r_picmip 1
r_mode 3
r_texturebits 0
cg_shadows 0
r_fastSky 1
r_inGameVideo 0
cg_brassTime 0
cg_bounceParticles 0
r_texturemode "GL_LINEAR_MIPMAP_NEAREST"
Fastest:
r_fullScreen 1
r_subdivisions 20
r_vertexlight 1
r_lodbias 2
r_colorbits 16
r_depthbits 16
r_mode 3
r_picmip 2
r_texturebits 16
cg_shadows 0
cg_brassTime 0
r_fastSky 1
r_inGameVideo 0
cg_bounceParticles 0
r_texturemode "GL_LINEAR_MIPMAP_NEAREST"