When precalcualtion is disabled, IK is solved from the slider's current position. For example, you will get two results at tick 50 when you move the slider from tick 10 to 50 and from tick 20 to 50. When precalcualtion is enabled, IK is solved when IK setup is done and the information is stored in the cache. You will always get the same result at tick 50. The slider's current position won't make any difference to the result because IK is just replayed once and once again.
When the precalculation sample is set to, for example, 20 and the timeline length is 100 ticks, then there is one precalculation tick every 5 ticks. Increasing the sample number shortens the distance between precalculation ticks that increases the accuracy of IK, but it will need more time for precalculation. If you have a complicated IK chain or fast moving goals, you will need higher precalculation samples for a better result.
The Precalculation settings can be found in the Bongo Document Options.