A synecdoche often uses a significant part of something to represent the entire thing. The phrase 'all hands on deck' uses 'hands' to mean the whole crew.
Pro Tip 2/3
Whole for Part
Sometimes the whole is used for a part, especially when speaking poetically or figuratively. The 'world' cheered for the Olympic champion, meaning people everywhere did.
Pro Tip 3/3
Cultural Awareness
Synecdoches can include cultural references that may not be understood universally. 'Hollywood' is used as a synecdoche for the US film industry.