Is there any trick to win in an arm wrestling or any training or exercises we can practice to excel in it?
Regardless of what the sources say, winning in armwrestling is mostly based on, at least from the perspective of the biomechanics of your arm, your wrist, hand, forearm, biceps, and chest strength. Supporting muscles are the shoulders, lats, and even triceps.
The key aspect to winning can (and usually is) be observed as a combination of skill and strength. In professional comp. most armwrestlers try to get an edge on their opponent based on varying grip positions and fast-twitch muscle fibers to get a leverage/torque advantage over their opponent:
Variations that can make or break good positioning can be: stance of your feet, angle of the difference between two arms(e.g. picture above clearly shows the woman on the right in a disadvantage), and body positioning(i.e. how close your arm is to your body can affect your lat's pulling ability).
While strength is key, speed seems to rule out as, if you've seen pro matches, the winner is usually faster, but may not exactly be stronger(e.g. applying torque a split-second faster, moving the arm out of comfort, and placing the opponent in a very delicate position is usually how most win).
All in all, a good indication of strength is not done with armwrestling. As one person loads stronger than another can lose to them at armwrestling, armwrestling is not a very accurate measure of upper-body strength(especially with so many varying factors).
There is an article on LiveStrong relating to arm wrestling.
As I have been asked to summarise it (in case the link goes dead):
Forearm strength and grip are very important. I would suggest hammer curls, barbell wrist curls, reverse wrist curls, dumbbell wrist curls and dumbbell reverse wrist curls. For your grip, you can get those hand grip exercisers.
Lock your elbow tight against your body after gripping your opponent. Your biceps and triceps stabilise your arm in this position.
When your opponent's hand is past centre (i.e. you are winning) rotate your body so it's positioned slightly outside your wrestling arm. This allows you to utilise your shoulder muscles, especially the posterior deltoid, to push down on your competitor's hand. Your (upper) back muscles contract and add force to your shoulder muscles, stabilising them as you rotate your body.
In addition, this is where your chest comes in (pectoral muscles). Tightening your abs will bring stability - hence why deep-lying core muscles are important and why I suggested something like plank exercises.