requiring you to be on first gear while descending must be for a good reason. i am not sure, but i guess being on first gear makes your speed on descent more controllable: you have engine break, and foot break when needed. no rapid desceleration the moment you release the break from half break or full break.

if you put your gear on neutral while on descent, your descending speed might be irratic and the possibility of you losing control of your car would be higher: hihina pag half-break, hihinto pag full break, gugulong ka naman bigla the moment you release your break (which would not happen kung naka-first gear ka) at dahil nabigla ka, magfull-break ka na naman.

on neutral, you have to descend on half break (timplahan mo ng maayos) throughout your descent on neutral. kaso lang, pagmawala ka sa focus at maiba ang timpla sa break mo, wala kang extra-alalay. either mag-overshoot ka or bigla kang magfull-break.

the tricky part in engaging first gear on descent is the fact that you have to step on the clutch before you step on the break; otherwise, mamatay makina mo. perhaps, this is the reason why you put your gear sa neutral.