Do "after update" trigger block the current transaction?

Hi,

We are currently evaluating the feasibility of executing long-running
scripts written in shell-script (plsh) called by triggers (after
update/insert) to synchronize two databases. Will triggers (after
update specifically) cause the execution of SQL-commands to pause
until the trigger-function has returned (at statement execution time
or commit)?

The other possible approach would be to use polling on some
trigger-updated timestamp-column, which is not pretty but should be
fairly simple.

Thank you in advance, Clemens