How JIRA Client Synchronization Works

There are several types of synchronization in JIRA Client:

Quick Synchronization. In this case JIRA Client in one request to the server downloads details for several issues selected based on some query. This download includes most of the fields, comments, work logs, links, lists of attachments.
Detailed Synchronization. JIRA Client does several requests to the server for a single issue. It downloads remaining fields (voters, watchers), meta data about editing, applicable workflow actions.
Small Attachments. JIRA Client downloads smaller attachment files - up to 100Kb by default. This size can be modified through auto.download.maxsize parameter in the settings.
Large Attachments. Download of the files larger than the specified limit.

When can each of these synchronizations happen

Quick Synchronization:

When a user runs a query which is not synchronized. At this moment all issues matching the query are downloaded (even those, which might have been downloaded as part of the other query). After the download is finished, the query becomes synchronized and JIRA Client just checks for updates.
When a users runs Reload Query From Server - all issues matching the query are downloaded.
If the automatic synchronization is turned on. JIRA Client periodically checks and downloads issues that were changed since the last check. To understand if there were any changes, JIRA Client compares last updated issue from JIRA with the last updated issue in the local database. If they match, no further actions are done, if not - JIRA Client downloads all issues which were updated since the previous check. The check runs every 150 seconds and this period can be changed via quick.sync.period parameter.

Detailed Synchronization:

When a user keeps Issue Details open for more than 3 seconds. Once the full details are downloaded, JIRA Client won't download them again unless the issue was changed.
On request - Download All Details. Can be applied to several issues at the same time.
On upload.

Small Attachments:

When a user keeps Issue Details open for more than 3 seconds. JIRA Client does not download attachments, which have already been downloaded.
On Download Attachments for selected issues.

Large Attachments: