Swaddling babies in snug wrappings has an ancient history, and is a practice that's now coming back into fashion in a more modern form.
Long ago, babies were swaddled in very tight bands that restricted movement for several purposes like keeping them warm, ensuring proper growth and helping soothe the children to sleep. Over time, however, the tightness of the binding fell out of favor and rightly so - babies spend nine months in a very small space and when they get out they want to move!
Too much wigglin' and jigglin' can cause problems, though, particularly if it results in baby getting turned around onto his or her tummy. Sudden Infant Death Syndrome (SIDS) is the leading cause of death for babies beyond the newborn stage and supine sleeping without loose blankets in the crib is known to greatly reduce the chances of SIDS.
Once you've got the baby dressed in sleepwear you then place the arms through the armholes, tuck in the feet and zip the wearable blanket closed. You'll notice that baby still has plenty of room to kick, but won't be able to kick out of the swaddle once the blanket is secured. This is ideal.
For an "arms in" swaddle, fold the left swaddle wing over the baby's right arm and tuck it under the left arm. Do the same with the right wing, but fold over both arms and secure it by fastening the Velcro pads to the garment. For an "arms out" swaddle, tuck one or both swaddle wings under the arms when you fold them over.
And so ...
The newly swaddled baby will have some range of motion while remaining warm without the use of loose blankets. Eventually, baby will be able to roll over and/or break away from the swaddling wrap and that's a sign that it's time to move her or him into a SleepSack wearable blanket.
The SleepSack swaddle and other products are used and endorsed by hospitals nationwide, and the swaddle has been approved by the International Hip Dysplasia Institute as "Hip Healthy" for babies.