It's best if you do crunches with interlaced fingers (at the webbing) placed behind your head. Elbows should be flared and away from your head. Back far enough that you can't see them. Keep them this way throughout the movement. Tara is doing a great demo HERE.
Placing your arms in this position will raise your center of gravity toward your head. This makes the move more difficult and places more tension on the abdominals during the crunch and adds benefit to the move. To scale, simply do the opposite. Move hands to the front and move towards navel.
Keeping the elbows back and out of sight also places more tension on the abs. Additionally, It gives you less leverage for pulling on your head and potentially straining your neck. Never pull on your head during crunches.
If you are in the Eagle Mountain area, come storm with GPP Fit Torrent.