A block of wax rubbed along a ledge or rail makes the edge slippery. Without it, trucks and deck catch abruptly — the most common cause of falls when grinding.
Wax sounds like a trifle and still decides whether a trick is possible at all. Rough concrete and untreated metal stop the trucks abruptly instead of letting them glide, and it is that jolt — not the grind itself — that throws you off balance. The typical beginner mistake is starting on an unfamiliar edge and blaming the resulting slam on their own technique. Rub the block along the edge several times until it feels smooth, then feel your way along it once slowly. Two caveats: porous concrete eats wax far faster than smooth metal, so you re-wax more often there. And wax stays behind at the spot — on a staircase or a pavement it turns into a slip hazard for pedestrians, which is why it belongs on the ledge and nowhere else.