What happens if you set prune= to a value between 0 and 550? Today I saw a question about what Bitcoin Core would do if you set prune=1. The documentation on bitcoin.org states: 


  A value of 0 disables pruning. The minimal value above 0 is 550.


Apparently, the log upon setting prune=1 states:


  2017-03-11 10:51:03 Block pruning enabled.  Use RPC call pruneblockchain(height) to manually prune block and undo files.


Will Bitcoin Core prune to the minimum disk usage now, or does it just enable manual pruning?

The asker was running Bitcoin Core 0.14.0, if it makes a difference.
It turns out that prune=1 is a new special case which enables manual pruning. For values of N from 2 to 549, prune=N shows an error.

Bitcoin Core parses configuration arguments in the init.cpp. Depending on the value of prune=&lt;N&gt;:


N &lt; 0 – returns the error: "Prune cannot be configured with a negative value."  
N = 1 – Activates manual prune mode as described in Pieter's answer. It causes Bitcoin Core not to advertise blocks anymore and enables the pruneblockchaincommand, but it doesn't automatically prune the blockchain data.
1 &lt; N &lt; 550 – returns the error "Prune configured below the minimum of N MiB.  Please use a higher number."
550 ≤ N – Sets N as the limit of MiB available for block data.