Mixin Accessors allow you to access fields and methods that are not visible (private) or final.

@Accessor allows you to access fields.

@Mixin(MinecraftClient.class)
public interface MinecraftClientAccessor {
    @Accessor("itemUseCooldown")
    public int getItemUseCooldown();
}

Usage:

int itemUseCooldown = ((MinecraftClientAccessor) MinecraftClient.getInstance()).getItemUseCooldown();

@Mixin(MinecraftClient.class)
public interface MinecraftClientAccessor {
    @Accessor("itemUseCooldown")
    public void setItemUseCooldown(int itemUseCooldown);
}

Usage:

((MinecraftClientAccessor) MinecraftClient.getInstance()).setItemUseCooldown(100);

@Provider allows you to access methods.

@Mixin(EndermanEntity.class)
public interface EndermanEntityInvoker {
  @Provider
  public boolean teleportTo(double x, double y, double z);
}

Usage:

EndermanEntity enderman = ...;
((EndermanEntityInvoker) enderman).teleportTo(0.0D, 70.0D, 0.0D);