As you know the AT90S1200 microcontroller includes an internal RC oscillator that is disabled by default. If you want to change it (enable or disable) you must to program it with parallel mode. The most programmers work on serial mode that is not possible to program this fuse bit (RCEN).
With this stand-alone programmer you can enable or disable the internal RC oscillator that works at ~1MHz.
The circuit is based, on the most of the parts and source code, to Mr Herbert Dingfelder’s rcen_blower (http://www.dl5neg.de/rcen_blower/rcen_blower.html).
For more detail: RCEN fuse programmer using AT90S1200A microcontroller