The injection is either syntocinon which causes uterine contractions that can force the placenta to peel off the wall of the womb. Then the blood vessels where it was attached shut off to minimise bleeding.
Some units use syntometrine which is syntocinon combined with ergometrine. The latter acts to clamp down the cervix. Hopefully after the placenta has been delivered - if it hasn't been removed quickly enough (through pressing on the belly and pulling on cord) then it causes retained placenta. Or cord snap.
Syntocinon is a synthetic version of oxytocin. It is given as an injection after birth for third stage and to (usually prophylactically) reduce bleeding. I have outlined the benefits of waiting before having it in previous posts.
Syntocinon is also given in much much larger doses via IM drip during induced or augmented labour. It causes contractions that are more intense but less effective than naturally produced ones- you need more contractions per cm of dilation. This is why it is generally administered alongside epidural, which usually slows labour further and increases chances of fetal distress and assisted/surgical delivery.
Syntocinon (when used during labour) is also an antidote to naturally produced oxytocin...
Oxytocin is the hormone the body produces naturally to cause contractions (uterine contractions, the let down contractions that cause expulsion of milk during breastfeeding and the contractions of orgasm - male and female too)
It is also responsible for feelings of love, bonding and intimacy in life generally - but is present in massive amounts in a mother and baby immediately after birth to start maternal love and bonding.
Unfortunately synthetic oxytocin (syntocinon) does none of these things - it's molecules are too large to pass across into the brain)
It inhibits production of natural oxytocin (which is why those who have laboured with it often look glassy or sunken eyed and shell shocked in those first few photos holding baby, where those who are high on natural oxytocin have large dilated pupils and look wide eyed (and feel as if they could conquer the world!))
Natural Oxytocin creates contractions which do not overwhelm our body, are manageable and effective. Syntocinon is labelled with a warning - may cause uterine rupture - the high doses frequently cause hyperstimulation which can lead to potentially fatal rupture. Which is why induction using syntocinon is especially dangerous for anyone who has had previous CS.
Natural oxytocin only produced efficiently by the body when we are feeling safe, warm and cared for. This is the same for all mammals (think about what a cat does instictively- finds dark warm safe place where she won't be interrupted)
This is because adrenaline is also an antidote to oxytocin. Any fear or anxiety will increase adrenaline, decreasing oxytocin and reducing contractions. This is where the stories of women in strong labour whose contractions slow or stop when they arrive at hospital come from.
Put simply, if you wouldn't make a baby somewhere, it is unlikely to be easy to birth one there without help unless you are protected, feel very safe and gently cared for.
You may be able to tell I am not a fan of syntocinon. It is generally a substitute for patience and good care - whether during labour or afterwards.
IMO it's use to stop heavy bleeding where it occurs post birth is it's only admirable quality 