I watch a lot videos of pros mounting and when they press down on the pedal the uni doesn’t roll back under them. Whenever I mount the uni always rolls back under me, how do you get it to stay “motionless” (best word I could think of)?
I haven’t been uniing long so I’m not an expert. But how I mount without the uni moving is I put my left foot on the pedal almost completely horizontal, just a little under it. Then jump up and forward catching the right pedal with my other foot. (when you jump up and forward your pedals should be horizontal by the time your other foot catches the pedal crank thing)
hold the wheel
do the same thing when you mount but hold onto the wheel when you mount. it will keep the wheel still.
I have been riding for a while and I mount without the wheel moving. Rather than stepping down on the pedal you should “walk” onto the uni and lean forward rather than lean back and have it come under you.
Don’t straighten your leg. Easy instruction, hard to do. But if you don’t unbend your leg the pedal won’t go anywhere. Give a push forward while not unbending the leg on the pedal, rotate up and over the top, and then start pedaling away with your other foot.
Repeat a zillion times until it’s second-nature. 
I start with one pedal all the way down. And looking at all the videos, that seems to be the wrong way to do it, but works for me.
You have to sort of push forward against the seat as you hop up onto the seat. That makes the uni want to move forward and away from you, and so the foot that’s on your pedal already, instead of moving downward, is simply preventing the uni from moving forward.
To gradually learn this, start by putting something behind the uni’s wheel–a curb, a 2x4, a rock–to stop it from rolling back when you mount. Then, move the wheel a little away from the object, and try to mount without the wheel rolling back and hitting the object. This has helped several people I’ve helped teach learn to do this mount.
Oh, it’s called a Static Mount.
Basically what John said, but the way I describe it is to lock your leg in that bent mounting position. So when you push up to mount by locking your leg in that position you’re not putting downward pressure on the pedal.
To get used to the feeling, try locking your leg and rock a little onto the Unicycle. Don’t go for a full mount, just try keep the unicycle static and step back down again. Keep repeating for a while. Then try do the extreme of that and step over the unicycle. Do the same movement as before but give yourself more of a push keeping the unicycle static and just step straight over in front of the unicycle. This will give you a good feel of where your balance point is. If you can manage that then stepping up into a mount should be relatively easy.
A method I used to start learning is to lean forward and grab the wheel with my hand to keep the wheel steady as I’m stepping up. Not great form but does work, and eventually also teaches you to lock your leg and put little downward pressure on the pedal. It did take me a while to get out of this habit though so I’d recommend the first method, but if you’re stuck then not a bad option.
Good luck, and just keep at it.
Why do you care anyway? I mean as long as you’re mounting fine who cares? The mounting stuff will come with time. For now spend time on what matters like learning neat tricks.
John and Johnny said it right
I’m only commenting because I am the absolute worst at this. I rode for months and wore out a tire before I tried to free mount. And then only because I had learned to idle, and upd in a rare spot with no hand holds. I used the step on the pedal, it goes down, then idle a bit and go.
I am now learning the magical "step on the pedal yet it doesn’t go down " tech. I think it’s called a static mount.
Mostly because I ride a 36 sometimes, and the static mount seems like the way to go.
So far, I just do it on the 19. I would say, don’t step on the left pedal. Think of it as having a wooden left leg that can’t step down. You are using your wooden leg to block the pedal, this keeps the wheel from turning, when you jump to the right pedal with your right leg.
im not a 100percent sure what you mean but when i get on i dont press down on the pedal so the crank goes to a vertical position like most new riders do. what you do is you have ti in the horizontal position and use really light pressure witht the foot thats on the pedal and step up then push forward.
It can be useful off road, for example, where you don’t have plenty of space to mount.
I found the static mount really easy, it just felt natural. However, I’m still working on the rollback mount (trying to idle). It’s really awkward.
After riding a while, I found out that with my dominant foot (right side), I could mount without putting the other foot on the pedal and hold it like that for a second or two. It’s about finding the balance between pushing the pedal down and “jumping” up.
My left foot wasn’t good at this so I trained it in using the method pkittle described with a little modification. Try mounting by putting little pressure on the pedal (it should by almost horizontal) and “jump up” on the saddle. Not enough eh? Put more pressure on the pedal and try mounting again.
Repeat these steps until you find the balance I talked about earlier. Then you can try to mount, hold it (don’t put the other foot on the pedal yet) and then put the other foot on the front pedal. It may be hard first but work on increasing the time you can hold it (I’m talking about 1-2 seconds here, not for a long time!) and I’m sure static mount will be like a breath.
Uni Geezer has a good vid on mounting as part of his youtube collection:
He makes it look so easy on the 36!