This is pretty much a form of indirect fire and one way the grenadier can help out when he doesn't have hand grenades like his rifleman buddies. It can be used in situations that would require a hand grenade like the enemy is over a hill and there isn't anything behind them to splash the grenades on. It's also good when the enemy has fortified a roof and made it extremely hard to go up to the roof such as watching the ladder or something.

Luckily, the grenade arcs are only different between the under barrel AK grenade launcher and the rest of the grenade launchers so it's not that hard to remember which factions require which adjustments.

First, get about 10 meters away from the target because that's the range that is easy to hit using this technique. This can be used at longer distances such as 100 meters but then trial and error will have to be used before grenades are accurate. Slightly longer distances like 20-30 meters can be also done just by carefully moving the sights down a couple millimeters.

https://youtu.be/fq9LqM9JMpM
Next, set your sights on the grenade launcher to 200 meters(make sure it reads 200m because sometimes it gets stuck at 50m) then aim straight up but also in the direction of the target. If you are using an AK with an under barrel grenade launcher(INS, MIL, RUS), aim about a millimeter lower on the screen or else the grenades will land back on you.

https://youtu.be/onyS7fMSuBI
If you are using most other grenade launchers(US, UK, AUS, and MEA), you will be relatively safe if you don't lower the sights. Of course, it's assumed there is some kind of cover between you and the impact area because the grenades will land about 10 meters away.

The arc is quite high so I'd imagine you could use this technique to hit the roofs on high buildings such as the ones on Al Basrah.

Very important: hold breath by pressing left shift because the weapon sway could lead to the grenades dropping back on you even though you didn't mean too. It takes about a half a second for the gun to actually come to a stop after holding breath. The grenades will still have some deviation because holding breath doesn't freeze the gun so be careful about that.

Also, hip firing does not work as it doesn't seem to take into account the 200 meters the sights are set to.

Sadly, this doesn't seem possible because the rockets either despawn when they get too high up in the air or they just don't slow down so this gives the grenade launchers an edge over frag rockets.

Canadian grenade launcher is difficult to use when shooting straight up. You will have to lower your sights after looking up about double of what you need to do for the AK launchers. It seems the CAF grenade launcher uses a different arc.