A slam requires a roll against DX or Wrestling to hit. The effects are resolved as a collision. Knockdown and knockback are resolved as for a collision, there are no additional rules for knocking your opponent down.
If you have one or more arms free, you can elect to grab your foe when you slam him. You both end up in the hex your opponent was knocked back into (if your opponent was not knocked back, you end up in the square he was originally in). Creatures with claws can cause normal claw damage while grabbing with a slam in addition to the slam damage as a separate wound. Characters with natural bite attacks can grab with their mouths - this requires a free mouth rather than a free arm. It causes normal bite damage in addition to the slam damage as a separate wound, but the biter takes slam damage to its face (for this reason many bite-oriented predators match speeds with their prey before slam/biting). If slamming with claws or a bite, attack using the higher of your Brawling or Wrestling skills.