Cryotherapy shrinks the capillaries, which slows bleeding and helps to prevent swelling. When you sustain a strain or sprain, tissue is taken well beyond its normal length and is often torn, at which point bleeding and swelling then occurs. This swelling will interfere with the circulation to the area and thus normal healing, so if you can do something at this point that prevents or reduces edema and inflammation it will definitely help expedite your recovery from an injury.

The earlier you address the swelling the greater your chances for an earlier recovery, and the easiest way is by applying ice right on the injury immediately.  Cryotherapy also prevents the spasms in the local muscles to relieve pain.

Cryotherapy for treatment of injuries dates as early as Hippocrates. Ways to apply ice are easier than  500 BC, but precautions and principles will always exist. Initially after the application of ice, the skin  feels cold, and this will in itself often relieve pain. As icing proceeds, you may experience burning, and then pain superficially, and at last numbness.

To prevent frost bite, stop icing as you start to lose sensation to light touch (not like  "numbness" one initially feels as cooling begins to relieve the pain. Keep applying cold after the pain diminishes) Applying ice for more than the recommended time can cause skin damage or nerve damage. Cryotherapy isn't recommended for all injuries as some people will not be able to tolerate it.

Icing time will depend on which method you use to apply it  and the location at which it is applied. Body parts that have little subcutaneous fat (like knees, ankles, and elbows) can not handle ice times as long as areas with more subcutaneous fat (like the thighs and buttocks). For areas with bone closer to the surface, apply ice for a shorter period as follows.

It's best to apply cold regularly  throughout your day, allowing for several hours between ice treatments. Time in between applications of ice will allow your skin and subcutaneous tissues to warm a little to a normal temperature. Ice bags remains the easiest cryotherapy treatment  for most, but other options are out there:

Bags of Ice

Pros: Bags of ice are the usual method of applying deep cold. Simple fill one or two  zip lock bags  with ice and wrap them in a wet towel. Apply right on the injury. The effect of zip lock bags full of ice lasts longer and has been found to penetrate deeper than other superficial methods such as ice massage. If ice isn't readily available, then grab a bag of frozen vegetables, wrap it in a wet tea towel and apply that.
Cons: Contouring the bag to the bumps and curves of your body for the best application may be difficult with large ice cubes. It molds better if you do not fill it full with ice or you can crush the ice instead. 
time: 10-20 min. This depends on the area of injury and level of comfort.

Gel Packs

Pros: Gel ice packs hold a gel that will freeze many times over. Keep these packs stored in a freezer to keep them handy.  The gel keeps its flexibility when in the freezer, thus letting it mold to the body part concerned.
Cons: Gel ice packs cool the cutaneous and subcutaneous tissues much faster than bags of ice and so require caution. Don't apply them right on the skin and always put them in a wet towel to conduct the cold.
time: 10 minutes, and monitor the body part for loss of sensation.

Chemical Bags

Pros: Chemical bags are stored at ambient temperature until the bag is squeezed and the chemicals are mixed. This produces cold. These are especially good on the sports field or when camping or hunting.
Cons: The temperature change of the bag is not as great as other methods, but the bags are good for first aid.
time: The temperature change produced is not as low; therefore, 30 minutes is acceptable, and this method can be directed right on the injury

Submersion

Pros: Submersion requires placing the injured body part in an ice water bath filled with ice. This provides complete  exposure to the whole injured part.
Cons: Only body parts such as the hand, foot, or elbow are suitable for this form of cryotherapy.
time: 10-20 minutes. Discontinue if this becomes too uncomfortable.

Ice Massage

Pros: Massage with ice requires you to rub ice directly onto the injury directly. This focuses the cold on the injured area and is easy to apply.  Often paper or foam cups can be used when filled with H2O  and frozen.  The top of the cup is then peeled away to expose the top of the ice -  the covered edge of the cup is held to apply the ice. Ice cubes may be applied as well directly by this method.
Cons: This method of cooling the tissues is not as effective as penetration is not as deep and the cold effect is not as long lasting as other methods described above.
time: Apply for no more than 5 -10 min., a little longer if the area has more subcutaneous fat.

Combination of R.I.C.E

To maximize cryotherapy, use RICE (rest, ice, and compression, and elevation).  To add  to cryotherapy, rest your injured body part, apply an elastic wrap firmly, and then keep your injured area elevated. New products combine RICE principles. For example, cold tape will compress the area it's applied to and due of a reaction of chemicals, cools the injured body part.


Remember you should ice early and frequently. Avoid skin and nerve damage by letting your skin recover between ice applications. Listen to what your body is telling you.

Heat may worsen inflammation and edema, so do not apply heat within the first 72 hours of an injury. Wait for inflammation to subside before applying heat.


When to Avoid Cryotherapy

Cryotherapy is not good for everyone. Those who are very sensitive to ice will not tolerate icing long enough to be effective. Also, those with a high tolerance to ice  may be prone to injury by applying cryotherapy for long than a safe period of time.

Those with circulatory issues should avoid cryotherapy, (i.e. Raynaud's disease - whereby the vessels in your hands, toes, nose,  and ears constrict if exposed to the cold or other stimulation).  If you're at risk because of you are diabetic or have other conditions that can affect blood flow, see your family physician before applying ice to a strain or sprain.