Potemkin rears back, then slams his massive body to the ground, creating a shockwave. This attack actually has two stages. The first is Potemkin's actual body, a standard attack that hits low and knocks the opponent down if they don't block it. Once he lands, the shockwave occurs: if the opponent is on the ground when it happens, they get knocked down. In both cases, the opponent is knocked down for considerably longer than usual, giving Potemkin plenty of time to close the distance, if applicable, and set up okizeme. Note, however, that if the first attack connects, even if it's blocked, the shockwave will not occur.
Slide Head has some upper body invincibility as it starts up, which might be enough to get you out of a counter-poke, but don't rely on it too heavily.
There is also a window in which Potemkin can hit the ground and create the shockwave effects, but then YRC before the actual shockwave occurs. Great if you're trying to scare the opponent into jumping.
You can also time Slide Head so that the shockwave hits just as the opponent is getting up. Only a reversal backdash, blitz, or another move with frame-one invuln/airborne state, will save an opponent from this. You can then RC, slowing down time and extending their knockdown animation, giving you time to combo them while they're still airborne and do some big damage.
There are some exceptions to the rules for Slide Head's shockwave. Some characters have dashes that are considered airborne, which allow them to avoid the shockwave simply by dashing forward (Slayer, Venom, Bedman, Ramlethal, etc.) Generally these dashes have obvious effects, like teleportation, or make a distinctive sound when starting up to indicate their unique status.