Bloons get pushed backwards or slowed after each hit.
~ BTD6 descrpiton
Knockback is the first upgrade of Path 3 for the Super Monkey in Bloons TD 6. It gives the Super Monkey the capability to push back or slow down bloons on each hit. The Knockback upgrade costs $2550 in Easy Difficulty, $3000 in Medium, $3240 in Hard, and $3600 in Impoppable. It will push back bloons besides Leads and Ceramics, slows down Leads or Ceramics (including their fortified variants) by 60%, or slows down MOABs and above by 30% (except the BAD, which cannot be slowed at all).
When looking for reliable sources of knockbacking, it is worth knowing how each of these cross upgrades interact.
3-0-1 Sun Avatar can knockback up to 6 bloons per sunbeam, 3 sunbeams per shot, and at double attack speed. Additionally, it can pop Leads but not Purples (unless Strike Down the False MK is active). It has the default Super Monkey range, though its range could be further increased with the upgrade Ultravision.
0-3-1 Robo Monkey can knockback up to 4 bloons per dart from its 2 separate arm attacks. It can pop Purples but not Leads. It has good range, and it can be further improved with the Ultravision upgrade.
0-0-3 Dark Knight can knockback up to 3 bloons per dark blade, can deal +2 damage to MOAB-class, and it can be crosspathed with other paths for additional benefit, preferably Path 1 with the double attack speed and +2 pierce.
As of Version 12.0, the nerf to the knockback towards Ceramics makes it difficult to stall Super Ceramics in place. In fact, the new knockback effect to Ceramics is now similar to that on MOAB-class bloons.
Dark Knight restores full knockback to Leads, Fortified, and Ceramics, but it is rather difficult to utilize Dark Knight for an intended purpose of stalling Super Ceramics.