With a very dense and fibrous cataract, using just the vacuum from the phaco tip to hold the nucleus is often insufficient to achieve the phaco chop and separation of the nuclear halves. In these cases, trapping the nucleus between the phaco tip and the chopper allows the use of much greater forces to disassemble the nucleus.
The key to this technique is to make sure that the two instruments are directly opposite each other and that the applied force vectors are aligned. This ensures that the nucleus will be trapped and immobile while we apply progressively stronger forces.
Click below to see how to trap a dense nucleus in order to chop it: