Why shouldn’t internal vibrators be forced down into fresh concrete?

Internal vibrators operate by generating impulses which liquefy the fresh paste so that the internal friction between aggregates is reduced. As a result of vibration effect, the mix become unstable and trapped air would rise to the top while aggregates would settle to the bottom.

Internal vibrators should be allowed to fall in fresh concrete under its own weight and should not be forced to drag down into concrete. The reason behind this is that the use of force to push down internal vibrators would leave a mortar channel in fresh concrete and this result in the formation of weak concrete along this channel.

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.