A defensive player may obstruct his opponent by using his body and/or arms. Strictly speaking, he is not allowed to strike, grab or tackle the opponent. However, as the usual penalty for frontal tackling or grabbing is just a free throw, defenders regularly try to stop the attacker by
– blocking/grabbing the attackers ball handling arm with one arm/hand while
– putting the other hand on the attackers hip/back and
– pulling the attacker against his own chest.
In order to keep the game flowing (thereby opening up holes in the defense) the attacker will try to evade this. For the same reason, referees usually do not interfere as long as the attacker is still able to pass the ball safely along to a teammate.