Mac OSX 10.10.4 introduces TRIM support for third party SSDs. Until then, you had to use third party tools such as Trim Enabler in order to get TRIM on your SSD.

By default, when replacing your HDD with a third party SSD (i.e. an SSD not from Apple), TRIM is still not enabled. But you can run a simple command from a Terminal which will do just that:

sudo trimforce enable

This will enable TRIM on any SSD that supports it. Note that you will have to reboot for the changes to take effect.

If you wish to disable TRIM (why on earth would you do that?), you can run the same command, replacing enable with disable.

Note that trimforce does not completely replace Trim Enabler as it only enables TRIM, and does nothing more.