Illustrations of history

Japanese use of gunpowder was more deliberate than the West. The cannon or musket wasn't loaded with so much gunpowder that the recoil would knock back the weapon - shots were intended to hit supposed weak spots in the fortification or armor.

Look at their firearm, the tanegashima, for an example. Look at their shoulders - there is no rifle stock to help transfer the shot into their shoulder.

It was a lighter load that was akin to samurai archery, a weak draw with intentional precision required. Quoted from the article:

Guns of the o-zutsu caliber (20 momme (≈ 75 g (2.6 oz)) and more) were practically portable hand cannons and were used as siege weapons employed to knock down the hinges of gates as well as powerful anti-personnel and anti-cavalry weapons.

So that wooden block in front of the wheel was probably meant to roll the cannon up to it without consideration of backwards movement because there isn't likely to be much.