0DTE options, mostly on SPX, SPY, and QQQ, have almost no extrinsic-value left and enormous gamma. Small index moves produce huge percentage changes in the option. They now account for a large share of index option volume.
They are lottery tickets for buyers and a theta harvest for sellers, and both sides can lose everything in an afternoon. Costs from the bid-ask-spread are large relative to the premium.
Example: at 10:00 a.m. an SPX call 20 points out of the money trades at $2. A 30-point rally by noon makes it worth $12, a 500% gain. A flat afternoon makes it worth $0.
Related: expiration-date, gamma, theta, extrinsic-value