To answer your question you would have to close the window once beyond 150' from the path, it would then go THR - VNAV SPD, the 1250FPM wouldn't apply immediately as it would be speed on elevator and be reducing towards the 300kt FMC speed, however once it got back to 300kts with the window closed assuming you never got within 150' of the path that initial THR application would now have the aircraft descending at approximately 1250FPM at 300kts in an effort to regain the path.
It appears the 1250 had been removed from the FCTM as it now just says "VNAV commands a reduced decent"