Over a 1M opening by partner, all jump shifts between 2M and 3M are artificial game-forcing raises:
(1) The highest JS (1H-3D or 1S-3H) is Swiss, 13-15 HCP, usually three-card support and a balanced hand.
Opener's rebids: 3NT is to play, 3 of his original suit requests Responder to bid 3NT (this allows Opener to position the notrump; true, he doesn't have much information to go on and won't always get it right, but at least he has the opportunity to guess instead of system making the decision randomly;he may also pull the 3NT rebid, showing a different kind of slam try from what the same bid would have meant without the interposed puppet), new suit is feature-showing slam try (but if you want these to be asking bids, splinters, or whatever, feel free), four of the trump suit is signoff.
(2) The next-highest JS (1H-3C or 1S-3D) is a GF raise with a long and strong side suit, nine cards in the two suits (four-card support and five-card suit or three plus six -- by analogy to "splinter," I call this a "Girder"). We also find it helpful if this guarantees two honors in Responder's suit, plus one trump honor, but if you want to be able to use this bid more frequently, you may wish to reduce the honor placement requirements.
Opener may relay (step responses) to ask the long suit (cuebids follow), or sign off, or bid a different suit as an "unless" signoff (= "Sign off UNLESS this is your suit").
(3) The third-highest JS (1H-2NT or 1S-3C) is a splinter raise with an unspecified *singleton* (old-fashioned double jump shifts like 1S-4D show *voids*).
Opener relays to ask, signs off, or bids something else as an "unless" signoff.
(4) The lowest JS (1H-2S or 1S-2NT) is a GF raise not strong enough to
take unilateral control (trotting out RKCB or GSF, or torturing partner
for a while via 2/1) and not right for more descriptive responses. This
will usually cover:
(a) a balanced hand too strong for "One-under Swiss"
(b) a balanced hand with a good four-card suit elsewhere,
where the other suit may be the better trump for slam, or
(c) a Girder (see (2) above) flawed in length (for Girders
flawed in honor placement, use 2/1).
(Other hand types may also be possible, but in practice we've always
held one of these three whenever we used this bid.)
Rebids by both Opener and Responder are natural/descriptive.
(This part of the Sea-Dog structure was inspired by the article "Features From the Top" in a recent _Bridge World_. Earlier versions of Sea-Dog used the lowest JS as a mini-splinter or, in a different version, the beginning of a master-slave relay sequence.)