The decomp is normally used to bleed off compression to get the piston passed top dead centre.
As God said Hondas normally have an auto decomp system that's worked from the kick start. I can only assume that the original system has failed or maybe someone has fitted an odd cam so a manual system has been fitted.
I'm told that some high lift cams can interfere with the auto system on Hondas.
On my XT500 Yamahas I push the kick start to get the piston approaching TDC on comp then open the decomp slightly and ease the piston just passed TDC than kick it. On a 250 it's probably not to bad but on a bigbore it makes it a lot easier.
hth
Brent