I can't speak to safe, but it's probably not very effective at encouraging crawling. I doubt there are any studies showing this, but intuitively it doesn't seem useful.
Babies will typically crawl when they're physically ready; strength, balance, etc. are all things that just take time. The main thing you can do for encouraging that along is 'tummy time', to get him/her comfortable with being on tummy and to let the baby start doing some strength exercises (ie, pushing up). You can also do similar types of strength exercises such as push/pull (where you push against the baby's hand or leg trying to get him/her to push back or similarly pull against pulling), or place objects near but not quite at the baby to encourage him/her to reach or try moving.
Some things definitely do not help with walking/crawling. Bouncers, for example, are net negatives if used too much, because they assist with balance too much so the baby doesn't learn to balance on his/her own. "Forced crawling" is probably not a negative as long as it's not done a lot (like an hour a day or something), but similarly it doesn't help the baby actually learn how to do the actions or use his/her own strength to do them.