A Tholian will cast a W-shaped web between his ship and an approaching opponent. In general, the web is placed so that the opponent cannot move through it during the four impulses it takes to solidify. Even if the opponent is not moving at a high speed, the Tholian must always be aware of possible speed changes which could potentially allow his opponent to move up to four times in the next four impulses.
The Tholian should look for opportunities to cast web when the opponent does not move on the next impulse. Because speed changes occur before direct-fire weapons fire in the Sequence of Play (webs are cast during the direct-fire weapons fire phase), this ensures that the opponent will be able to move, at a maximum, only three hexes before the web solidifies.