All transactions on sidechains will be trimed periodically(every 120 blocks),and only save the hash of snapshot on blockchain(materchain).
The snapshot just record the status of current sidechains without history transactions.And all nodes are updated,so they can generate such a hash themselves.
Before trim,all materchain nodes need check all current transactions and hash of history snapshot are valid,then such a trim will be implemented.
New nodes need to download and check the all hashes of snapshot when downloading blockchain data,and will be much faster overall,maybe in several minutes.
Masterchain can not be trimmed for securing network.