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Prevents the target from switching out. The target can still switch out if it is holding Shed Shell or uses Baton Pass, U-turn, or Volt Switch. If the target leaves the field using Baton Pass, the replacement will remain trapped. The effect ends if the user leaves the field or if the user or target faints. Note that if the user then uses Baton Pass, the effect will now end. Pokemon protected by Magic Coat or the ability Magic Bounce are unaffected and instead redirect this move back to the user. Does not prevent the effect of Eject Button. If the target is hit with Roar or Whirlwind while Block is in effect, they will be forced out as per normal.
Spider Web isn't very useful. The two spider Pokemon that exist, Ariados and Galvantula, are very frail and can't afford the time or the moveslot for this move. Sure, the trapping effect is nice, but the distribution really holds it back.