The video above shows aspiration before an intramuscular injection.
After inserting the needle but before injection, the plunger of the syringe should be drawn back for 5 to 10s. This is to make sure the needle is not in a blood vessel.
If it is in a vessel, blood will appear in the syringe and the needle should be taken out.
(Thomas, Christine M et al., 2016) (Summary)
Only inject when there is no blood in the syringe.