Share stock
Sharing stock, also known as life stealing, is the action of transferring a remaining life to a fallen teammate who has no more lives. This puts the character back into action (at 0%), with one life, and removes one stock from a team member. This is initiated, by the defeated player, by pressing Start/Pause (A+B in Brawl) when out of lives. Naturally, this can only be done in a team stock match, where a teammate has more than one stock remaining. In Brawl, the narrator will announce "Share stock" when this is done.
If a team has three players, stock is taken from 1P, or in 1P's case, 2P. Assuming that the primary stock giver has been defeated, stock will be taken from the remaining teammate.
It is wise for team members to discuss or ask permission before stealing a life, not only as it may be considered rude not to do so, but for strategic purposes. If you were to steal a life while a teammate had two stock, but a high damage percentage, you could essentially be swapping your life for theirs, as they are likely to be knocked out soon afterwards. Of course, communication is not always available during online matches, so sharing stock in these instances may be more of a personal decision.
If you are using "Share stock" for strategic purposes you should also take into account the stage you are playing at. For example, if you are playing at a stage similar to Hyrule Temple your partner should be able to stay alive until very high damage percentages, while if you are playing at a stage such as Final Destonation your partner may die at fairly low damage percentages. Another factor to take into account is how heavy or how lightweight the character your partner is playing as is. For example, someone like Bowser will be able stay alive longer than a character such as Olimar. You should also consider the skill level/CPU level of your partner and the opposing character(s).