The reading is based on information gained in the bidding and the play to previous tricks.
[1] The technique is used by the declarer and defenders primarily to determine the probable suit distribution and honor card holdings of each unseen hand; determination of the location of specific spot-cards may be critical as well.
This means keeping track of one small number, and your own trumps do not enter the calculation.
For example, 5-3 and 4-4 are among the most common trump distributions on the declarer and dummy's hands.
In cases, if an opponent shows out on the second trump round, then 5-3-1 or 4-4-1 is known, and the pattern 5-3-4-1 or 4-4-4-1 comes up automatically, and the other defender is known to have begun with four.