Provisional cast-ons are used to keep live stitches along the cast-on edge that can later be picked up and worked, grafted, etc. To work it, you need the needles called for by your pattern, waste yarn, and a crochet hook in a size similar to your needles.
Setup: Make a slip knot near the end of your waste yarn and place it on your crochet hook, keeping it somewhat loose. Hold your knitting needle and the waste yarn tail in your left hand with the working tip pointing away from you and the end of the tail toward you.
Casting On: *Cross the crochet hook on top of your knitting needle. Take the working yarn under the knitting needle and up and over the crochet hook from the left side. Catch the yarn with the crochet hook and pull the new loop through the old loop. You will have added one stitch to your knitting needle. Rep from * until you have the number of stitches called for in your pattern on your needle.
Finishing: To prevent the cast-on from unraveling while you knit, add a few extra chains with the crochet hook only: **Ignoring the needle, simply catch the working yarn with the crochet hook and pull the new loop through the old loop. Rep from ** until you have an inch or so of crochet chain tail.
If desired (recommended), run a lifeline through the stitches you cast on. This can make picking them up again later, especially if you are new to this style of cast-on.
The following video demonstrates how to work the provisional cast-on:
When you're ready to undo the provisional cast-on, start from the side with the long crochet chain by pulling gently on the yarn tail. This will free up stitches one by one. As you remove the waste yarn from each cast-on stitch, pick up the resulting stitch from back to front with the needles indicated in your pattern. If you placed a lifeline in the first row, follow that as you pick up stitches. You should end up with the same number of stitches on your needle as you cast on, but if you end up up with one fewer, just pick one up from the edge when you get to the end.
The following video demonstrates how to unzip the provisional cast-on: