Upper and light leathers are generally empty in nature and so the fiber bundles can slide one above the other when the leather is bent. This sliding effect improves the flexibility, softness and feel of the leather. Heavy retanning reduces the flexing endurance of the leather. Chrome tanned leathers are more empty than the vegetable tanned leathers and so the flexibility of the former is more than the latter unless the vegetable tanned leathers are given an extra fat treatment.

When a leather is flexed or folded several times as in the case of vamp portion of the shoes, bellows of portfolios etc the free grease is pushed away from the flexed region so the leather cracks there or wrinkles develop.

In the Bally flexometer a leather specimen of dimension 70*45mm is folded and fixed to the two jaws of the instrument in such a manner that the grain side remains outside with a fold on the specimen. The motor is switched on when one clamp remains fixed and other one moves backwards and forwards causing fold in the specimen to run along it. The leather is thus flexed in the folded condition. The number of flexes or cycles at which the first sign of crack of the finish layer appears is noted by periodically observing the test specimen. If the test is allowed to continue, The crackiness of the finish layer would be followed with a crackiness of the grain and finally the complete failure of the leather itself.

To get this result the present day light leathers require flexing of 80,000 to 1,00,000 times or more. So, in practice two leather samples are collected from different leather pieces of different lots are generally compared by flexing the both for 1,000 ; 2,000 ; or 5,000 times only. Any type of change in the finish layer is then compared.