"In the pocket" is a phrase you'll hear a lot of fighting/boxing announcers use referring to a fighter who stands within the range of his/her opponent to slip strikes and counter in close combat. It's strategic -- and dangerous -- for several reasons.
Hold your hands in front of you like you're blocking phantom punches. Now extend one hand out in a slow motion punch and freeze when you're hand reaches maximum distance.
Being "in the pocket" would be one voluntarily positioning hisself/herself between these two points, which prevents the opponent from creating maximum velocity on strikes while being in the position to create short-velocity strikes. A good fighter can maneuver "into the pocket," release rapid fire punches and duck back out without suffering much damage.
The dangerous part occurs when an unskilled fighter continuously tries to slip into the pocket against a more skilled opponent, taking a lot of abuse and never getting within optimum range -- quite possibly looking like a wind-up toy that keeps bashing into the wall because it's essentially programmed to do one thing.
Roller derby definitely has an equivalent to "in the pocket" -- and can often be seen when players enter the pack.
Good hitters need a lot of room to maneuver. Force equals Mass times Acceleration. The bigger the skater times (speed and room) creates optimum velocity when meeting an opposing object. Getting "in the pocket" of an opposing blocker can remove a great deal of her advantage, but the disadvantage comes when smaller skaters try to get "into the pocket" of a bigger player.
Much like the inexperienced boxer in the example above, a smaller skater will take massive amounts of abuse to get into the area where she can be most effective. All too often we see the smaller players get obliterated by well-timed hits.
The beauty is when a smaller skater gets position and controls the bigger, stronger player.
This happens for two reasons: The main is that the smaller skater has now removed a large part of her opponent's effectiveness by limiting her acceleration (F = M x A) -- by limiting the acceleration she has limited the amount of force. The second reason a smaller skater can control a larger player when "in the pocket" is center of gravity. A taller, bigger skater has a higher center of gravity than a smaller, shorter player.
By that right, it's easier for a smaller skater to get under the center of gravity of a larger player sometimes, but not always, resulting in moving the larger skater. (Sometimes this results in the larger skater falling on the smaller one with some hilarity and concern for the player on the bottom.)
To get "in the pocket" of an opposing player, it's advised to get beside to a little in front of the player.
It's not advised to do a song-and-dance with trumpets trumpeting all while wearing a clown costume.
You're best weapon is surprise and stealth. Slinking in an out until you can manuever yourself into position without taking the brutal, crowd pleasing hits, is your optimum goal.
If you're a short player, use your size to your advantage. Skaters have blind spots, just like cars do. By positioning oneself behind a larger skater, you can essentially disappear off their radar and when they least expect it, you pop back into view already in position.
A smaller skater can also use other players as screens, maneuvering around a friendly to position yourself in front of an opponent. The major downside to this is getting caught in a domino-like 2-for-1 hit: Your opponent hits the friendly, who inadvertently hits you -- thusly eliminating two players with one hit.