One or a few patents are used to protect an invention. Depending on the patent scope such a patent may block a whole new technology as in the case of a pacemaker invention. However, pacemaker inventions rarely happen and the scope of the patent is limited by the prior art.

A single patent may block a competitor to use the same invention. If the patent scope leaves space to maneuver the competitor will try to design around the blocking patent or to invent a substitute of equal value to the costumer. Depending on the technology and the magnitude of the blocking effect the design around may give the patent holder a lead time from a a few month to some years. In the event the patent holder is able to use this lead time to establish a dominant design or to generate a lock-in effect, even the imperfect scope of protection of a single patent may be turned into a de facto monopoly.

A patent may become a strategic patent, although small or moderate in scope if is able to block a technical field completely. Think for example of a composition of chemical elements for an alloy with radically improved properties, such as high magnetic coercivty and remanence. If not known before in the state of the art or anticipated by the person skilled in the art, even a small range of the claimed chemical elements renders this patents powerful, as the products outside the range, the leftover for the competitors, do not show the surpring effect.

Grandstrand termed it "patent blanketing", but you will find also expressions such as "patent thicket", "patent mine filed", or "patent cluster".

A patent thicket is set up by a patent owner, who wants to block all possible ways to design around his product, that is already protected by a patent.  A patent cluster may have a slightly different meaning. For example to protect a car a car manufacturer will have patents on the motor, the steering, the wheels, the saftey belt, etc. 

As it is a very expensive strategy a patent thicket is only justified if the returns made with the protected product is quite huge. A recent example is Gilette's razor with five stacked razor blades. In their own ads they claim that this five bladed razor is protected by more than 20 new patents [compared to the three bladed razor].

If money is an issue a single line of defence might do the same job as a patent thicket. Special attention has to be dedicated to constantly watch the fence for revealing loop holes and close these loop holes systematically.

Once a strategic patent position has been lost to a competitor it can be more effective to limit his freedom to act to the boundaries of his own patent.

Surrounding the patents of the competitor with patents for useful extensions may not hinder him to exploit his patent, but may hinder him to extend his inventions. A situation that may end in a draw, which may force the competitor to enter into a patent exchange program.