I use CanMoney, but with a similar interface, so trying to clarify.
'Hold' is a facility to "HOLD' / "Block" the of money specified by you in your linked bank account and will make available those funds to your trading account so that you can go long / short in your chosen instruments. When you HOLD some amount, the said amount is not available for cash withdrawal, but is available for trading.
Same way, when you RELEASE some amount, the said amount is reduced from the amount available for trading, but cash withdrawal from your bank account is increased.
At the end of the day, the actual net amounts will be credited / debited to your bank cash balance / trading balance.
murthymsr