http://concretepumpingvideos.com/videos/1067/air-blows-concrete-pump-hose

No its not.  I admit to using air from time to time.  I have figured out the safest ways possible to do it.  But I still dont like using it.  Last time I did, it was raing, it was late, and I just needed to get out of there.  End of the hose clogged, and me being in a rush, didnt let the pressure off.  Cleared the clog and then I relized what I did.  No one was around the area, just 2 guys left in the truck on the other side of the site.  Well it let loose, and out it came.  I was 22 feet down in a hole and my ENITRE truck was covered in concrete, along with the surrounding area, and me.  I came close to taking a 2" hose in the face.  I aint doing that agian.  

Don't use air, even if you know what your doing.  

Use air is scary. Its kind of like those guys who stick their head in a alagators mouth for fun. DONT STICK YOUR HEAD IN A ALAGATORS MOUTH FOR FUN.

We got a schwing 20m with an 800 liter watertank. I look forward to blowing a ball with water but never got the chance yet. Blowing ball with air is scary. Specialy if you are so dumb to let someone hold the end of the hose in his hands!!! 

If I do it I let the hose lay by itsself. Sometimes even secure it with a rope and let everyone take distance just in case. Most of the time I mannage to let the ball come quite gently out of the hose but you never know... If you realy f*ck up propperly the effect is beyond all imagination

Okay I fall into the guilty category...may have used air once or three times.  I'd like to hear some opinions on the following use of air cleanout...

put the ball in section farthest from the pump, stroke pump in reverse with hopper lid closed and use air GENTLY to help push the ball back through hose into hopper...

I've done this quite a bit, especially on "no mess" jobs and works great, just have to make sure the pump is actually in reverse before feeding air to it or it tends to build pressure super quick!

Is this safe?

Only concern I would have is how fast the ball woud fly threw your pipe after the hose and if your hopper can hold it all.  Its ok to suck back a ball threw hose.  If I am running off the tail of the pump I use a small ball to clean out hoses if its a wetter mix.  They are saying dont crank up pump and blow the concrete out of the hose with air. 

Nice hard end on the hose, he is lucky he has his teeth.

He is not using air. he is dry stroking the hopper so he can suck back a sponge. The hose man was lucky that he didnt adjust that hose when that burst came through because he would of got it good.