Hair Conditioner is the Yin to your Shampoos Yang...it refortifies the cuticle with a protective coating, allowing the hair to maintain suppleness and elasticity - this helps it to stretch and not break easily.
When your hair is exposed to the outside world's environmental pressures, plus styling your hair with heat and tension, the cuticle, or the outer protective scale, gets damaged. This creates an 'open door' allowing moisture to come and go easily (porosity) and exposes the hair's cortex (inner protein bonds) to these outside stressors until it ultimately breaks; the conditioner fills in those outer gaps in the cuticle and coats the hair to prevent damage to the cuticle. A damaged internal structure can ultimately result in breakage.
How to Use Hair Conditioner
Believe it or not, there is a right and wrong way to condition your hair.
The right way is to shampoo your hair at least once, but even better would be twice. Rinsing thoroughly each time. The squeeze out excess moisture so you don't dilute the conditioner too much.
Then apply a small (ten cent piece size, more depending on your hair length/density) of the conditioner in long, milking motions. Begin at the ends and work your way towards the roots. If you feel your hair needs more product then go ahead and add more.
* Pro Tip - wide toothed comb can be your best friend to evenly distribute the conditioner.
As far as how long you should leave it in for? Two minutes should suffice, as this is approximately how much time it takes for the conditioner to work it's magic on the hair. If you're using a deep conditioner, however, he says three to five minutes is ideal.