You should try this :
Code:
public func(id) {
new Float _velocity[3];
entity_get_vector(id, EV_VEC_velocity, p_velocity)
if ( p_velocity[2] < 0.0 ) {
client_print(id, print_chat, "[AMXX] You can't make checkpoint while falling");
return PLUGIN_HANDLED;
} // Else the player is just standing or Jumping
}
But this not very advanced, because if a player is just going downstair or something like that it can handle because Z-velocity goes under 0
In my opinion it should be enough for you that's it !