Walking with trekking poles provides stability and can give you a better all over workout. A single pole will be very useful when crossing loose terrain or fording moving water but using two trekking poles reduces strain on knees and ankles while increasing walking efficiency by using upper body strength to increase the forward thrust of your walking technique.
Nordic Walking is the ultimate example of this involving vigorously pushing your self along using your poles at an angle behind you, the thrust of energy from each arm corresponding with the “push off” from the opposite foot. This technique helps channel more of the energy in the foot push off into forward momentum rather than a less efficient up/down motion.
This varies according to individual preference but generally have your arm flexed at around forty five degrees and keep your poles shorter when going uphill and longer when descending hills. Going uphill with two long poles will reduce the input from your arms and coming down with short poles will strain your back. Adjust your poles as your walk progresses and if you need both hands to scramble put them away on your sac.
Most people need a pole that can be telescoped or dismantled and packed inside a sac at some stage. The standard three piece trekking poles are ideal for this.