Only by convention and precedent. FDR's threats to pack the court certainly influenced them and if implemented would have subverted them.
FDR couldn't implement that by himself, though; the bill had to be approved by Congress.
Convention and precedent (and tradition) are also forms of hierarchical power. Those who assert them in the course of making a decision exercise power over those who intend to deny them.
SCOTUS <> White House <> Congress
That's the 3 branches of government with its checks and balances.