When you shrink a partition, you lower its size to make freespace on your harddisk. This freespace can be used to create another partition.
Partitioning is dividing your harddisk into one or more partitions.
The first technique doesn't erase your existing partitions (however, it is not without riscs).
The second technique does erase the partitions that are touched.
The safest way to proceed is buy a new harddisk and disconnect your existing harddisk while installing on the new partition.