Minecraft – Summon Sideways FallingSand

First create a clock with the following commands:

execute @e[name=TPSlimeBack] ~ ~ ~ tp @e[type=Slime,name=SlimeKnock,rm=1,r=2] @e[type=ArmorStand,c=1,name=TPSlimeBack,r=2]

And:

execute @e[name=TPSlimeBack] ~ ~ ~ /effect @a[r=2] minecraft:resistance 1 100

Then summon an invisible ArmorStand and an invisible Slime with a size of 1 on the block where you want it to knock the player back by doing:

/summon ArmorStand ~ ~ ~ {CustomName:TPSlimeBack}

And then on the same position:

/summon Slime ~ ~ ~ {CustomName:SlimeKnock,Size:1}

This will make a Slime that knocks the players back, but that can't move out of the block's radius because it teleports back to the armour stand all the time. The player also get's a resistance effect to make sure no one loses their lives.

Edit

A much easier way to do this is just to summon a Slime on the block with the Size of 2 and NoAI added. Do this by running the following command:

/summon Slime ~ ~ ~ {CustomName:SlimeKnock,Size:2,NoAI:1}

(Note: The custom name isn't really needed, just to kill the slime when you want to remove it.)

Ok, after some (5-10hrs) work I got a solution:

Now, what the numbered parts are:

1. Button switch - one button press is like a lever switch
2. Fast Redstone clock (minimal repeater delay)
3. Piston that turns the system on; the repeater is essential, otherwise the piston will go on-off-on on button press
4. Command blocks, each part has these four commands:

/execute @a[ry=112,rym=-112] ~ ~ ~ fill ~ ~ ~+1 ~ ~+1 ~+1 minecraft:barrier 0 replace minecraft:air

/execute @a[ry=22,rym=157] ~ ~ ~ fill ~+1 ~ ~ ~+1 ~+1 ~ minecraft:barrier 0 replace minecraft:air

/execute @a[ry=-67,rym=68] ~ ~ ~ fill ~ ~ ~-1 ~ ~+1 ~-1 minecraft:barrier 0 replace minecraft:air

/execute @a[ry=-158,rym=-22] ~ ~ ~ fill ~-1 ~ ~ ~-1 ~+1 ~ minecraft:barrier 0 replace minecraft:air

5. Finally, this last Command Block resets the whole cycle:

/execute @a ~ ~ ~ fill ~-5 ~-2 ~-5 ~+5 ~+2 ~+5 minecraft:air 0 replace minecraft:barrier 0

Notes:

• all repeaters must be set to minimum delay
• the lower #4 command blocks are not connected with a repeater, the upper ones are.
• all the Pistons are Sticky Pistons
• the system will not delete the remaining Barrier blocks when turned off; It's better to use this:

• I recommend to use "minecraft:stained_glass 4" instead of "minecraft:barrier 0", if the player breaks it, a new glass block appears almost instantly.
• It's convertable to use the 1.9 chain command block feature, but I don't recommend it because of different repeat frequency.
• The system is reliable - you can't move forwards whatever you try.
• Even if you are falling/flying upwards/sprinting/potion sped up, the barrier block(s) farther from you will be deleted.
• You can try it yourself and use/test/edit it! - let's say, it could look like this:

The Command block next to the Button has this inside:

/clone XYZLeftChest XYZLeftChest XYZWool

The other two new Command blocks reset the cycle:

/setblock XYZChestUnderHopper minecraft:chest

/setblock XYZWool minecraft:air

Inside the left Chest place some items for signal length.