This mini-convention was originally written up in the March 1996 Bridge World. I think it's a fine idea.
Doorknob is a double or redouble only used by the opening side in auctions that start:
1C (1H) 1S
1D (1H) 1S.
So basically, we've opened a minor, they've overcalled a heart, and our partner bid a spade (showing five or more). When right-hand opponent doubles, or bids 2C/2D/2H, we can use an artificial double or redouble to show our handtype.
Most 2/1 players use Support (re)Doubles here, but I've never found that too useful. When I have a minimum opening and know we have an eight-card spade fit, I want to get to 2S ASAP without giving the opponents more room for discussion. So until I read this article, I used double or redouble to show a good non-directional hand.
Kleinman suggests using a double as showing the common handtype that's the toughest to show here-- a good five card minor with two-card (generally honor-x) spade support. This follows the 'Support Double principle' in that a double shows a known seven card fit for partner's suit.
In several years of use, this convention has come in very handy several times, getting us to strong 5-2 fits in spades and good 5-3 fits in the minor. The one time that the Doorknob double got passed, it was a great board too. Both partners knew there was a misfit, and knew their partner's source of defensive tricks.
As with all new conventions, we need to think about what we're giving up. In this instance (assuming we're already playing Support Doubles), all we're giving up is responder's knowledge that opener has three or four spades for the raise to two. I really don't think this is a problem though I realize some Law of Total Tricks nuts do. For you Law folks-- when opener raises to 2S under Doorknob, responder knows that there's an 8 or 9 card fit, but opener knows the exact length of the partnership's fit, and can LOTT-compete as necessary. Oh, and read Anders Wirgren and Mike Lawrence's great book I Fought The Law Of Total Tricks.