When you enter a boss room in TP, there is always a cs before the fight starts (either immediately after entering or tied to a trigger). If you skip a cs in TP it often reloads the area with a fadeout to avoid bad looking transitions. But since the area gets reloaded, the cs would play again in an endless loop. To avoid this the programmers used flags and upon skipping the cs the flag gets set to a high number and quickly counts down to 0. If the area gets loaded again and the flag is not 0 the fight will be started immediately. Same happens if you void. If you reset during such a void in a boss fight the flag can't count down to 0 and remains at a high number. The flag is not changed to 0 upon soft resetting either, only a hard reset clears it.
Skipping the sacred grove 1 (EMS) or second grove 2 (before tot) cs sets the flag to 1 without changing it back to 0 due a programming error in the way the sacred grove is handled (if you void or die in the grove the area state gets set back to the beginning and you have to howl again).
/!\ Be careful, if you you do a hard reset it cancel the glitch /!\
Having the flag at 1 or higher results in a lot of early bossfights (KB 1 or sacred grove cs can be used so far). However most of these early bossfights are not useful due to the time it takes to set-up.
For now there seems to be no timesaving use for Early Boss fights in TP HD.