Boost spiking is defined as a momentary over shoot in boost pressure over the desired boost level. A boost spike is the result of the boost controller holding the signal to the wastegate actuator / external wastegate for too long. Clamping the signal to the wastegate can increase the response of turbo system and reduce lag by effectively keeping the wastegate shut and not allowing it to bypass any exhaust thus not wasting any exhaust energy.
A spike in pressure can be a desired effect as it gives the engine a large increase in midrange torque without placing a lot of strain on the engine as the higher boost pressure is only reached momentarily at mid rangeRPM rather than having that boost level all the way through the rev range. Some cars from factory have boost pressure spikes to make the engine feel more responsive and powerful in the midrange.
A wastegate needs to be given time to “catch” the boost curve. To eliminate a boost spike, the wastegate must begin to open before target boost by a certain amount of time depending on the turbo system. If the wastegate begins to open at target boost, a boost spike will occur as the wastegate will need some time to open to the right valve height to divert enough exhaust gas to maintain the desired boost level.
(Taken from http://www.turbosmart.com/technical-articles/boost-spiking-explained/)