I didnt make OpengGL so i cant tell you the inner workings - however...
HI_RENDERTOTEXTURE_MODE_COPY - Is the mode that was first in there (render to backbuffer and copy)
HI_RENDERTOTEXTURE_MODE_FASTCOPY - Is done using glCopyTexSubImage2D which is less acurrate (sometimes) but a bit faster then using glCopyTexImage2D
HI_RENDERTOTEXTURE_MODE_DIRECT - is done using FBO's or framebuffers which should be the fastest out of the lot but isnt as wildly supported.
Just google the terms for more info.
HI_RENDERTOTEXTURE_MODE_COPY - Is the mode that was first in there (render to backbuffer and copy)
HI_RENDERTOTEXTURE_MODE_FASTCOPY - Is done using glCopyTexSubImage2D which is less acurrate (sometimes) but a bit faster then using glCopyTexImage2D
HI_RENDERTOTEXTURE_MODE_DIRECT - is done using FBO's or framebuffers which should be the fastest out of the lot but isnt as wildly supported.
Just google the terms for more info.