Which map is active when I overwrite text with R

In insert mode, I bind TAB to a custom expand function that is based on hippie-expand. However, when I overwrite text with the R command, I would also like to be able to use the expand function. During overwriting, TAB is bound to indent-for-tab-command and unfortunately it's not clear to me which map is active during overwrite. Could somebody please give me a hint? Thanks! Titus