JQL search for an approver, not in the default Approver field

To get the pending approvals of a user with JQL I do:

Approvals = pendingBy("an.email@addre.ss")

This works fine if the approver is added to the default “Approver” field of an issue. But how can I search for pending approvals, if I chose a custom field for approvers in the workflow?

I saw an example that looked like:

Approvals[Approvals] = pendingBy("an.email@addre.ss")

which also returns the pending approvals, but I didn’t understand the syntax.