Scripting Reference
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Rigidbody.AddRelativeTorque

function AddRelativeTorque (torque : Vector3, mode : ForceMode = ForceMode.Force) : void

Description

Adds a torque to the rigidbody relative to the rigidbodie's own coordinate system.

As a result the rigidbody will start spinning around the torque axis.

JavaScript 
    // Spins the rigidbody around its own y-axis
    function FixedUpdate () {
        rigidbody.AddRelativeTorque (Vector3.up * 10);
    }
using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void FixedUpdate() {
        rigidbody.AddRelativeTorque(Vector3.up * 10);
    }
}
import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def FixedUpdate():
        rigidbody.AddRelativeTorque((Vector3.up * 10))

function AddRelativeTorque (x : float, y : float, z : float, mode : ForceMode = ForceMode.Force) : void

Description

Adds a torque to the rigidbody relative to the rigidbodie's own coordinate system.

As a result the rigidbody will start spinning around the torque axis.

JavaScript 
    // Spins the rigidbody around its own y-axis
    function FixedUpdate () {
        rigidbody.AddRelativeTorque (0, 10, 0);
    }
using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void FixedUpdate() {
        rigidbody.AddRelativeTorque(0, 10, 0);
    }
}
import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def FixedUpdate():
        rigidbody.AddRelativeTorque(0, 10, 0)
If you want to apply a force over several frames you should apply it inside FixedUpdate instead of Update.