Translocation is the movement of sucrose and other organic materials from one place to another within the plant body, primarily through the phloem.
Concentration (pressure) gradients drive this process. 
In general, this happens between where these substances are made (the sources) and where they are used or stored (the sinks). 
Translocation typically begins in any plant location where sucrose is in high concentration.
There is a lower sucrose concentration in the sieve tubes (of the phloem).
This concentration or pressure difference causes the sucrose to diffuse toward and into the phloem of the leaf and hence to the phloem of the stem (where there is even less sucrose).