Laser guns calculate a speed by sending out a series of pulses. Each pulse reflects off the target vehicle and back to the gun, and by measuring the "time of flight" of the pulse, the gun can obtain a distance reading. By taking a series of distance measurements over time, a speed reading can be calculated and displayed.
Laser jammers work by first receiving pulses from the gun, and then timing the jamming pulses in such a way that the jamming pulses are accepted by the gun as it's own reflected pulses.
There is a very narrow time window to get a jamming pulse to the gun when it will be accepted and used for speed calculation. The laser guns are "gated", and they'll only accept a pulse and use it for speed calculations when they're expecting to receive a pulse. The guns are smart enough to know that each time they send out one pulse, they should only receive one pulse back. Thus, the gun is only expecting to receive a pulse from the time it sends out a pulse, but before it receives the reflected pulse back. Once the gun receives it's reflected pulse back, it isn't expecting a pulse until it sends out another pulse, etc. Jamming pulses must be received in this narrow time window, in order to have any effect on speed calculation. Pulses received outside of the time window are basically ignored, not used for speed calculation by the gun, and thus no jamming effect.
For example, a Kustom ProLaser III sends out 200 pulses per second, or one pulse every 5000 microseconds. At 1000 feet, the "time of flight" of the gun's pulse is around 2 microseconds (2000 nanoseconds, about 1 foot per nanosecond for total travel of 2000 feet). So, any jamming pulses must be received by the gun in that 2 microsecond window. If a jamming pulse reaches the gun during the next 4998 microseconds, it has no effect at all and is essentially ignored by the gun. Then, the cycle repeats for the next pulse.
To get jamming pulses into this time window, the jammer first receives a few of the gun's pulses, with which it can calculate the pulse rate and thus "predict" when the next pulse will be received. Then, it can send a jamming pulse (or series of pulses) a number of nanoseconds before it expects to receive the next pulse from the gun. The jamming pulse makes it back to the gun before the gun's own reflected pulse, and is accepted by the gun as it's own reflected pulse, is used for speed calculation.
Once the time window is calculated, the jammers might use a variety of techniques against different guns. Jammers recognise individual guns based on the pulse rate, this is called using a "look up table". With a look up table, jamming can be customized for the specific laser guns. This is done so that pulse timing can be custom-tailored for each laser gun for optimal jamming, and avoiding jam codes. For example, perhaps the most obvious application is that each jamming pulse might have random delays, so that the gun receives several different distance measurements that do not coincide with any consistent speed, so no speed is displayed. Another example is that the jammer can "fake" error conditions that can occur in normal targeting situations, causing operational error codes to be displayed on the gun which aren't associated with jamming. In addition to a "look up table", some jammers use a default algorithm to attempt to jam unrecognised guns. Not all jammers use a default algorithm.
There was an older jamming technique, used by the Lidatek LE-10 and Target LE-850. These units do not precisely time their pulses like todays jammers do. These jammers send pulses at a very high frequency of 4 MHz (yeah I know the patent says 2 but I was told 4 by the Lidatek engineer). This means that there is a jamming pulse every 250 nanoseconds. Since light travels at roughly 1 foot per nanosecond, this means that at least one jamming pulse would make it into every time window and be accepted by the gun, as long as the vehicle is more than 125 feet away from the gun. Once the target vehicle is closer to the gun, say 100 feet, the time window would be only 200ns, and since jamming pulses are sent out every 250ns, they might miss the time window so some pulses wouldn't get jammed. This scheme should successfully jam most guns. But, there are some drawbacks. This scheme will definitely cause jam codes on the guns. And, the laser diodes are not rated to operate at 4 MHz, but the developers found that they could operate at this frequency for a very short time without burning up the diodes. So, jamming is only possible for a short period of time, generally a number of seconds. After that, the laser diode must cool before it can jam again.
Jamming every single pulse from the gun isn't always necessary, it only needs to jam enough pulses to prevent a speed reading. Although in theory a speed could be calculated with only two distance measurements, in order to be truly accurate the gun needs a number of consistent pulses to display a speed, generally 50+.