Bursts are used to allow higher data rates for a short period of time. Every 1/16 part of the burst-time, the router calculates the average data rate of each class over the last burst-time seconds. If this average data rate is less than burst-threshold, burst is enabled and the actual data rate reaches burst-limit bps, otherwise the actual data rate falls to max-limit or limit-at.
Let us consider that we have a setup, where max-limit=256000, burst-time=8, burst-threshold=192000 and burst-limit=512000. When a user is starting to download a file via HTTP, we can observe such a situation:
At the beginning the average data rate over the last 8 seconds is 0bps because before applying the queue rule no traffic was passed, using this rule. Since this average data rate is less than burst-threshold (192kbps), burst is allowed. After the first second, the average data rate is (0+0+0+0+0+0+0+512)/8=64kbps, which is under burst-threshold. After the second second, average data rate is (0+0+0+0+0+0+512+512)/8=128kbps. After the third second comes the breakpoint when the average data rate becomes larger than burst-threshold. At this moment burst is disabled and the current data rate falls down to max-limit (256kbps).