This is something that can only be done with a very low ping and decent hardware.
Essentially you press W and target the enemy and then immediately press Q while you are traveling to complete the Headbutt.
This will cause Alistar to execute his Pulverize as soon as his Headbutt animation has finished.
Because the enemy has a travel time from the Headbutt, the Pulverize will interrupt their movement and instead knock them up into the air.
If you ever see an Alistar seemingly activate his Pulverize and pound the ground for no reason, it is most likely because they attempted to perform this combo but the target was out of range.
gaming.stackexchange.com/users/20923/RovingBlade
right as you headbutt you quickly pulverize and instead of getting to far away they go up into the air.
Its like if malphite ults in a straight line and alistar is in that line not at the end of it and pulverizes right when malphite is on top of him he will interrupt malphite's ult and malphite
will just end up being pulverized.
gaming.stackexchange.com/users/23061/Ryan McGahee
to instant headbutt>pulverize you headbutt your target and immediately after press pulverize.
just remember that this creates a quick and unsuspected CC and is less efficient than headbutt + pulverize or headbutt into a wall > pulverize.
Instant headbutt > pulverize creates an opportunity for a quick gank if your jungler is very close, or creates a kill opportunity for your AD carry (assuming they have CC like vayne's condemn)
gaming.stackexchange.com/users/23051/herpderp101
To add more information to the previous answers, you pulverize while you are in motion of headbutt.
If you are smartcasting, you'll want to be careful to be in range of the headbutt, so you just don't pulverize the ground.
gaming.stackexchange.com/users/22971/iLevin
Do not try to do it with smart cast it is very important to know the exact range of your headbutt so that you can time it correctly.
Nothing makes teammates more angry than someone knocking someone to safety that they were about to kill.
Try the Smart cast range indicator in the more options in your main menu it should help you learn the range.
Once an Alistar has practiced enough it becomes second nature.
gaming.stackexchange.com/users/22416/GGClassic-Jona52
I believe you should Smart Cast on it.
There was an idea that by sequencing smartcasting fast enough you'll be to get it off without timing. http://vimeo.com/23622202
gaming.stackexchange.com/users/23127/LookingAtYouKid07
Saw the video post and brought a youtube one for quick play
gaming.stackexchange.com/users/23133/user156133
You can do this with ping even around 150.
The idea is to simply press your Q as soon as you see Alistar begins his headbutt animation.
This will basically put his Q into effect as soon as your headbutt hits the enemy, so they will jump ever so slightly before getting a knockup.
You should note that if you are going to headbutt the enemy into a wall, it's better to do so.
They will be "CCed" for longer as they have to wait about half a second for the travel time they would have went with headbutt.
You can walk two steps towards them then pulverize to get the full effect of the knockup and stun.
gaming.stackexchange.com/users/22193/user22193
Here's Xpecial from Team Solo Mid (TSM)
playing alistar.
If you watch it, it's a good way to see timing for the combo and how to play Alistar:
gaming.stackexchange.com/users/23228/lostpuppydog
Watching the pro players, whenever someone plays Alistar they somehow chain pulverize and headbutt in an instant. Does anyone know how they're doing that?
How do players link Alistar's Headbutt and Pulverize in an instant?
gaming.stackexchange.com
<league-of-legends>
gaming.stackexchange.com/questions/60619
gaming.stackexchange.com/users/22120/Enix Blade ggClassic
You press W and target the enemy then press Q to complete the headbutt. You pulverise while you are in the motion of headbutt. The combo works if the player finds the proper timing while using the skills.
It is explained how to perform the hotkey combination.