How do Piston samplers function?

In sampling clays or silts, Piston sampler is lowered into boreholes and the piston is locked at the bottom of the sampler. This prevents debris from entering the tube prior to sampling. After reaching the sampling depth, the piston is unlocked so that the piston stays on top of the sample going into the tube. Prior to the withdrawal of the sampler, the piston is locked to prevent the downward movement and the vacuum generated during the movement of the piston from the sampler’s end aids in retaining the samples recovered. As such, sample recovery is increased by using Piston samplers.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.