It supports extended history and yes the plugin has special code for that feature. The requirements are:
- you have to use 64-bit version (eSignal stopped supporting 32-bit long time ago)
- you have to be subscribed to paid option "extended history" on your eSignal account.
- the checkbox "Use Extended intraday request" must be turned ON (in File->Database Settings->Configure)
- your database has to be configured with significantly large Number of bars (way beyond default 500K bars), see: How to increase max. number of bars (Database settings)
- after you change the Number of Bars setting for existing database you have to do "Force backfill"
- please do the math about how many bars you need as too much is bad too see http://www.amibroker.com/guide/x_performance.html