Can I use this sign for failing a test?
Yes, this is the perfect sign for failing an exam or a class. If you are talking about receiving the specific letter grade 'F,' you can also just sign the letter 'F' on your palm, but this sliding 'fail' sign is very common.
Does it matter which hand does the sliding motion?
Yes, it does. In ASL, your dominant hand should always be the one performing the active movement—in this case, the sliding and falling motion. Your non-dominant hand serves as the stationary base.
Can I use this if my computer or car fails?
While people will understand you, ASL usually relies on different signs for mechanical or technological failures. If a car or computer stops working, it is more natural to use the sign for 'break down' or 'die' rather than this sign.