Regular shackles work where the eyes they connect must or may be parallel to each other. Twisted shackles are for where they are normal to each other.
For example, the loop on the mast hoop that supports my throat halyard is verticle - the shackle loop going through horizontal. The block I use has it's attachment non-swiveling fitting such that a pin goes through normal to the shiv's axle. So a regular shackle would have the block lay against the mast with one side - hoist or haul - bearing on the mast. To orient the block to lay with it's cheek against the mast would require either two plain shackles (clunky) or one twisted.
Twisten or plain, you use what it takes to give a fair lead.
G'luck
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