what car are you using? and also whether it's auto/manual?

You will need to read the car manual to find out how and when to engage and maintain the lower gears.

There is either a S or L on the gear shif. (Aside from P, R , N and D - that is Parking, Reverse, Neutral and Drive.)

This S or L will prevent the car going into the highest gear. This is the preferred driving mode when going uphill and downhill as lower gears give the car more power as well as higher engine braking.

In some cars, there is also the O/D Off button on the gear stick. This "overdrive off" button is off in default mode. Meaning the car will go into overdrive mode for better fuel efficiency.

When you pressed the button and the O/D Off is lighted on the dashboard, it means overdrive is off and it will not go into the highest gear. This maintain the car in lower gear and maintain power when going uphill. And provide more engine braking when going downhill.

It's called the O/D OFF button. Press the button, it will cut off overdrive. Normal driving, you want overdrive for better fuel efficiency.

Manual keep it just gear 2-3 with brake on and off, dont press it all the down you might lose it from overheating. Automatic cars, the same, switch to 3-2 if there is such option, or D.

D is normal drive, computer shift gear according to speed. Higher speed = higher gear. You don't want that going down hill. Downhill must have engine braking using low gear, some AT car can go down to 1, but nowadays probably only down to 2 only.

Up slope with D, computer sense not enough torque then will auto drop gear to give torque / acceleration so revs go up.

All normal unless for whatever reason you drive until red line

So, meaning, going downhill with steeper slope, AT car needs to shift to lower gear eg. 2 or 1, right?

Recently, I experience AT car going downhill at Cameron Highland with D Gear, the RPM shot up and car like want to rush forward (lost control)...

Once at 30km/h release acceleration and let it role with D will do too when almost reach 60km/h brake slowly until reach 30km/h and let it rolling again

Last time I drove X70 up the hill, don't even need to put low gear. Turbo settle all the way.

But downhill, need to put low gear.